Why can the US invade Iraq for weapons of mass destruction but not North Korea who have confirmed nuclear weapons? Are they more scared of NK?
Because there's no oil in NK.
And there's the Sino-North Korean Mutual Aid and Cooperation Friendship Treaty, which would make a war with NK a (nuclear?) war with China.
WMD were only part of a complex reasoning that led to the Gulf Wars. The West had no serious problems with Iraqi WMD as long as they were aimed at Iran. But then Saddam Hussein miscalculated and went after Kuwait, and suddenly Iraq was a "rogue nation." The enduring hostility from the unfinished 1991 Gulf War led to the 2003 regime change. (Wikipedia overview)
Several nations other than the "official five" have developed nuclear weapons -- Israel, India, Pakistan, South Africa, North Korea -- yet there were no wars to disarm them. Others "without" nuclear weapons programs are far closer to the bomb than Iraq or Iran ever were -- Germany, Japan. More have chemical and biological weapons.
A cynic might argue that North Korea has too many WMD to disarm them, and Seoul under their (conventional and chemical) guns.
A realist might argue that Iraq was a gross miscalculation regarding the ease of regime change, and there is no appetite to repeat that mistake before the Middle East is settled.
Why can the US invade Iraq for weapons of mass destruction but not North korea who have confirmed nuclear weapons?
It's not true that the US invaded Iraq because of Iraq's supposed WMD capability. Saddam Hussein had already gotten rid of his WMDs to end sanctions. He didn't publicize the fact that he had done so, but the US government knew that he had. Hans Blix's team found the facilities completely disused. One site was covered in a "three-inch layer of pigeon dung."
Rather than WMD's, the root cause of the war was a sense of overweening power in the Bush administration. People like Madeleine Albright had already been pushing for regime change, and during the Bush administration many others in the foreign policy establishment had been saying you have this military power, why aren't you willing to use it to overthrow Hussein? Bush was being told that the Iraqi people would welcome the US as liberators, and that an invasion would lead to a democratizing cascade in places like Iran. It was made clear to people working in the intelligence agencies that their reports should support the case for war. The US public was still angry about 9/11, and although Iraq had nothing to do with the attacks (most of the hijackers were Saudi), there was emotional fuel for a war. Colin Powell was finally pressured into making a presentation of the falsified evidence to the UN, even though he had initially thought the idea of invading Iraq was ridiculous and obviously wouldn't happen.
The question is written in the present tense, but the situation with North Korea was very different in 2003, the year the Iraq war started, than it is now in 2020. North Korea didn't have a functional nuclear weapons capability then. If the US had invaded in 2003, it would have quickly overwhelmed their military, and the only immediate retaliation would have been artillery attacks against Seoul. The question would have been the reaction from China. The situation could have easily spiraled out of control. Eliminating the North Korean nuclear capability would have required a US occupation of the entire country, and it's very unlikely that China would have allowed that to happen without acting.
An invasion today would be extremely risky for all of the same reasons as it would have been in 2003, but in addition there would be the risk that North Korea would be able to use its nuclear weapons (and possible other WMD's) as an effective deterrent against the US, US military bases in Asia, or US allies such as South Korea and Japan.
Re Albright, see Mazarr, https://www.jstor.org/stable/24907218
Because the relevant weapon of mass destruction in that case was money, not things which splode.
* that shortly before the US went into Iraq, Iraq had moved to primarily selling their oil in Euro's. The US befits from USD being backed by oil / the de facto reserve currency (think Breton Woods 2.0) And to be fair, for the most part, it works quite well for the rest of the world as well.
So even though NK has some nasty splodes. They are only posturing, not credibly disrupting things; particularly money. Further more, NK is aligned to China. If the US were to make moves on NK, that would not feel comfortable for China. Remember how the US felt, and reacted, when the USSR was looking to set up shop in Cuba!
* I don't recall where sorry.
Basically North-Korea is close to being an unofficial protectorate of China. Invading North-Korea would probably get China to declare war on the US. You also would have to ask yourself what it would do to South-Korea one of the few bastions of democracy in Asia.
A lot of South-Korea still harbour ambitions of a unified Korea and not only would it lead to great turmoil on the geo-political landscape in Asia, it would probably kill of what small chance there is of unification.
The United States just can't invade the North Korean Regime because of the nuclear strength of North Korea. If Washington and its army want to bring down such a regime it wouldn't be impossible that North Korea might use nuclear force to defend its sovereignty and in that case, would not only be a disaster in the Korean Peninsula but also across the Continent and American Allies. It's even worse taking the risk, as such a strike from Washington needs strong military strength and would require military mobilization in South Korea and bases across the region. North Korea also has support from China and Russia.
- Why can’t the US attack North Korea like it did Syria? The Korean Peninsula technically remains in a state of war. Fighting halted on July 27, 1953 under an armistice signed between Washington and Beijing. If the US initiated an attack, it would break the treaty endorsed by the United Nations.
- China would have to help North Korea during American Ambush as both signed Sino-North Korean Mutual Aid and Cooperation Friendship Treaty and China wouldn't want American influence and interference in its near region and it's not unlikely that Russia wouldn't play business.
China is concerned that its border provinces would be inundated with North Korean refugees if the Kim regime collapsed. From a geopolitical point of view, Beijing views North Korea as a buffer zone from the potential encroachment by powers are aligned with the US, including Japan and South Korea.
The official reason for the invasion was that Iraq was not complying with the conditions of the cease fire that ended the first Persian Gulf War. If you wish to have it explained why the US did not enforce an agreement that North Korea was similarly violating, you'll have to say what that is.
As far as practical motivations, that's arguably off-topic, as it requires mind-reading. North Korea posed more of a threat, it has less strategic value, it has strong ties to China, and those all could have been factors.