It's not the number of casualties that scares people, but rather the nature of the threat.
Fires have existed for several millennia. Our ancestors who built and lived in the very first settlements suffered from their homes/stores occasionally burning down. We know what types of conditions increase risk of fires and we know how to minimize those risks and put the fires out when they occur.
Bombs on the other hand are unpredictable. They also cause their damage instantly and there is no way to minimize or prevent it. You can escape from a burning building, or if stuck, wrap a piece of wet cloth around your mouth to minimize the amount of smoke you breathe while you wait for rescue. You can't outrun an explosion.
That's why people are a lot more scared of bombs than they are of fires (or car accidents, for that matter, which kill many more people than both fires and bombs combined).
Availability bias is definitely one aspect, but I think a big part of it is also how easy it is to tell a story that separates oneself from the victims (this often takes the form of victim blaming, but not necessarily). It's easy to tell yourself the story of how heart attacks happen to people with different lifestyles or genetics, or how car crashes happen to drivers who are less attentive, or how violent crime happens to people who live in other neighborhoods. It's a lot harder to tell yourself the story of how you'll avoid the plane with the latent mechanical fault or how you'll never be at a gathering place that would make an attractive terrorist target.
it seems to be an unfortunate emergent behavior of groups of humans.