Even nuclear meltdowns aren't' enough to literally destroy the planet. In the worst case they might kill a few thousand people and cause billions in economic damage. Which sounds like a lot, but on the scale of the whole planet is almost nothing.

All of them are real dangers, and present dangers at that. But they still are not existential dangers. Even in the worst case humanity wouldn't go extinct. It's unlikely even modern civilization would collapse, just a few countries.

AI is one of the few things that can literally wipe out humanity, or even the entire universe. So I think it's worth special consideration. Personally I also believe it's far more likely as well. And people are already doing things about climate change and nuclear proliferation. Until very recently, there were only a few people working on AI risk, and it's still nowhere near enough.