One would think that if the US military were to leave Afghanistan, they would surely take any equipment that might fall into the wrong hands, especially since there were reports that the "wrong hands" would take over after the fall of the Afghan military and government.
What caused them to leave in such a hurry that they would even risk leaving dangerous equipment behind?
Some of the equipment was a bit more sophisticated than the "small arms" mentioned in the William Walker III's answer. I think that's also what the article in the question is referring to. Much of that equipment wasn't left by accident, it was given by the Americans to the Afghan forces, for them to work with.
For example, one article in the Guardian quotes a Russian arms exporter claiming over 100 military helicopters have been captured by the Taliban. Per the Guardian article:
As the Taliban overran the Afghan army and took control of large stores of arms and vehicles, it also captured at least 100 Mi-17 Hip helicopters, a Russian-made transport aircraft procured by the US for the Afghan armed forces because it was comparatively cheaper and easier to fly than US-made UH-60 Black Hawks.
The US had shifted to providing Black Hawks to Afghanistan in recent years, in part because of restrictions on working with Russian weapons manufacturers and exporters of the Mi-17. But far fewer Afghan crews had been trained to maintain the aircraft. According to Sigar, the readiness of the Black Hawk fleet fell by half to just 39% in the period from April to June as aircraft maintenance contractors were pulled out.
The main problem with more complicated equipment like aircraft is that they require regular servicing and crews familiar with operating them. As US contractors have withdrawn from Afghanistan in July of 2021, most of the complicated US equipment will be useless soon. According to NBC News:
The Afghan security forces rely heavily on U.S.-funded contractors to repair and maintain their fleet of aircraft and armored vehicles and a whole array of other equipment. But the roughly 18,000 contractors are due to depart within weeks, along with most of the U.S. military contingent, as part of Washington's agreement with the Taliban to withdraw all "foreign" troops.
Without the contractors' help, Afghan forces will no longer be able to keep dozens of fighter planes, cargo aircraft, U.S.-made helicopters and drones flying for more than a few more months, according to military experts and a recent Defense Department inspector general's report.
Of course, that reasoning goes for the Taliban too. They cannot use much of the equipment without training and servicing (which includes parts and technical knowledge). The reasoning from the US perspective may have been that US contractors could go back to Afghanistan if the Afghan armed forces were able to keep control of the equipment. Or they could guide Afghans trying to service them over Zoom. Now that last part may sound funny, but that's an actual suggestion from US defense officials as reported by Politico in July of 2021:
As the withdrawal continues, more of that wrench turning will be done by Afghan crews, with U.S. contractors looking over their shoulders via Zoom or coaching them over the phone, defense officials say.
And it was also the US plan for the gifted equipment to be used (of course not by the Taliban), as the Politico article goes on to quote Pentagon spokesman Kirby about the Afghan Air Force:
“They've got capacity. They've got capability. They have an air force — an air force, by the way, that we're continuing to fund and support,” Pentagon spokesperson John Kirby said Friday on CNN. “They've got modern weaponry. They've had training and the ability to be in the field with American forces much over the last 20 years. ... Now it's time to have that will."
Over the past decade, the U.S. has built an Afghan air force modeled on its own strengths and preferences, spending $8 billion to deploy strike aircraft such as the A-29 Super Tucano and AC-208 Combat Caravan, both of which are propeller-driven planes that can fire laser-guided munitions at ground targets. The U.S. has also sent new Black Hawk helicopters.
The maintenance problems are less of a problem for simpler equipment like armored vehicles and guns. As an expert is quoted in an NPR article about the equipment left behind or given to Afghan forces says:
For vehicles like the up-armored Humvees, known as MRAPS, "they've captured so many of them that they could cannibalize the ones they have for spare parts to keep the others running," he says.
Clearly, some of the more complicated equipment was not supposed to fall into the hands of the Taliban. While they may not be able to use it themselves, they may be able to sell it to US adversaries who can then study US equipment. As the NPR article continues:
And the Taliban can always just sell off anything they can't learn to use or maintain themselves.
On the Black Hawks and A-29s, for instance, "presumably there is some avionics, communications equipment, other things on those aircraft that they could sell," Bowman says.
Iran might be interested, as might China or Russia, if for no other reason than to "humiliate America," he says. Despite the sectarian divide between the Sunni Taliban and Iran's Shiite government, there are some signs of cooperation
Schroden agrees, pointing to high-tech "sensor balls" on the front of some aircraft.
"They have sophisticated electro-optics, optical equipment, as well as signals intelligence type stuff in them," he says. "Those things might be of interest to other countries as well."
Based on the above, my hypothesis is that the US didn't expect for much of the equipment to fall into the hands of the Taliban so quickly.
Because they were ordered to. Having been deployed I can tell you it takes time to pack everything up and ship it out, especially when there is no sea port to move massive amounts of equipment. It's not just throwing it into a container, you have to pack and secure it properly to prevent damage, you have to abide by environmental laws and regulations, safety regulations, do inventory, mark hazardous material, and move it to its point of debarkation. As I mentioned there is no sea port so only so much stuff can be shipped at once while trying to move personnel, and the Taliban take over made it impossible to plan to ship everything overland to Kabul (which by the way would be almost impossible for some sites as they were brought in by helicopter over a period of months to years because of location in the mountains). Also, keep in mind not everything the Taliban took over was left behind by the U.S. military. Some of it was sold or issued to the Afghanistan military and subsequently abandoned.
One would think that if the US military were to leave Afghanistan, they would surely take any equipment that might fall into the wrong hands...
One would be naive to think this.
Nearly every time the United States has withdraw from a military deployment, large amounts of weapons and other materiel are left behind - some captured, much of it US issue. This happens because the replacement cost for things like small arms can often be less than the cost of hauling it all the way back home at the end of the mission. Oftentimes this equipment is abandoned, at times outright sold, by our allies in the theatre.
Some examples:
The list goes on. The only military equipment the United States goes out of its way to prevent falling into enemy hands is stuff that has current-generation technology in it, and even then they tend to destroy rather than recover.
You can debate whether or not this is wise, but that it is happening is neither unique to Afghanistan, nor solely a function of excess haste.
The US wished to give and be seen to be giving the Afghans every chance of standing up to the Taliban. Removing the equipment that has been provided to the Afghans defence force because the US think the Afghans defence force has no willingness to fight would have resulted in an instant win for the Taliban.
Hopefully there is already a plan being put into action to destroy any equipment that the Taliban may sell/give to other country or use agaist the US.
However remember the Taliban has been most dangerous to Western countries using the most basic equipment.
It's often the case that equipment is not cost effective to be retrieved.
For example equipment that would need to be dissembled and then reassembled after air transport will often need to be refurbished and re-certified before it can be used again.
The facilities required to do this are limited, and equipment would need to be stored and may be in storage for a couple of years while it waited for a slot. By which time it would be both obsolete or simply cheaper to buy a new model.
Plus equipment may be contaminated with things such as dust from depleted uranium rounds or chemicals, and while these things can be allowed on the battleground or in open air environments, they would not be acceptable in a warehouse stateside.
After the first gulf many vehicles that fired DU rounds needed to be stripped back to the chassis and rebuilt. This is affordable for a main battle tank costing millions, but not an artillery piece.
