What is it about these ridiculous apocalyptic scenarios that makes them so appealing to conspiracy theorists?

None of them ever turns out to be true, so they can't be solely devised by propagandists or people interested in using them to mobilize public support or influence opinion.

There must be a deep-seated psychological need in some people to know that they are helpless, endangered, and that no human action or rational plan can ever help them. My guess is that this is how some people justify the need for religious belief, or at any rate, some supernatural force, that will come in and rescue then from these inevitable calamities. The idea that we can contribute to our own salvation is anathema to the truly devout. They are desperate to be forgiven.

On the other hand, precautions against genuine dangers with realistic potential solutions, like nuclear war, environmental degradation, climate change, or diseases for which vaccines and hygienic measures exist, are bitterly resisted. Its as if the recognition that any human action should not be relied upon because it threatens the whole concept of divine intervention. Don't trust scientists, they are all atheists, and God will punish us for taking their advice.

Of course, religious arguments alone cannot deny reality forever, so additional logical reinforcements are called in using cultural, political or even economic themes. This is how religious orthodoxy is inevitably enlisted to promote regressive and authoritarian policies. And it only works on an ignorant, but reverent, population.