The USSR could attack anywhere in the US on a minutes notice. North Korea can barely even attack Japan.
If North Korea were to use a nuclear weapon they would be annihilated by the US, even without considering any allies that may join to stop a serious nuclear threat.
With the USSR, if one side launched nuclear warheads, it was a given the other side would as well. It would have literally been the end of the (western) world as we know it.