North Korea's army is far larger than Iraq's. Plus South Korea doesn't really want reunification because it would ruin them. These factors are more important than nukes in explaining the difference.
This has been discussed before and I do sympathize with the South Koreans. But unless China take over North Korea, unification is beyond anyone's control. When -- not if -- NKR collapse, where will the people go ? China certainly does not want them. These are Koreans. North and South are temporary entities. When East Germany collapsed, did West Germany had any say in the matter ? Where could have the ordinary East German go ? No doubt a lot of East German elites fled to Russia and no doubt a lot of NKRean elites will be granted asylum by China, but where could ordinary NKReans go ?
The signs for a NKR collapse is more assured than the many 'US collapse' threads on this forum. Given how surprised I was when the Berlin Wall came down and later the Soviet Union broke economically and politically, and when I was active duty at that, now I fully expect to see a NKR collapse in my lifetime. If China steps in and take over NKR, that still would count as a collapse.
The economic cost to China in the maintenance of a buffer state will stress the Chinese government and create resentment among the ordinary Chinese. If a lot of Americans do not care for a US military presence in SKR, there will be a lot of Chinese who will go beyond 'do not care' to outright righteous anger that their tax yuan are used to support a puppet government in NKR, rehabilitate a failed economy, and to feed a starving NKRean citizenry.
Before the Korean War, the Chinese government can put up somewhat a facade that there were hundreds of thousands of Chinese 'volunteers' eager to fight the American imperialists. That kind of deceitful propaganda -- Glorious Socialist Revolution and the extolling of Mao Thoughts -- will not pass today.
The painful choices for China are:
- Make North Korea truly a vassal state under absolute Chinese control.
- Allow reunification and lose a geopolitical buffer state.