At least in part I think it’s because everyone dies eventually but we tell ourselves that we live on through our family and community and we also rely on family and community to support those left behind when death happens; so (all other things being equal) we care significantly more about children dying than old people even where both are preventable, and we also care more about mass deaths which affect a single community very acutely than mass deaths spread out across many communities.

We also think about causes in quite different ways again precisely because everyone dies eventually. Shortening of life is ‘less bad’ than preventable death which is ‘less bad’ than deliberate killing.

For nuclear disasters you can then layer on an element of irrational fear of the unknown and lack of control which sets it apart - the horror of ‘this thing I don’t understand might kill us all in a few days and we can’t stop it’.

For the same reason regular cars that kill e.g. 1 in 10,000 in accidents (as we know them), are more acceptable than a design that might be far more safe individually, but when in the (far more seldom case) one blows up, it kills everybody in its 2km periphery.

Even if the total death count is the same (or even if its less for the new design), the widespread small scale pattern is more acceptable than sudden mayhem.

For one, you could just not get in a car, and avoid recklessly crossing the street in the first case, and you'll be fine. In the other case, in the off chance of a new-car blowing up, it can destroy your whole 5-10 block periphery. I wouldn't accept that...