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 As US contractors have withdrawn from Afghanistan in July of 2021, most of the complicated US equipment will be useless soon.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.