Isn't the scale of potential damage far different between the two? A failed nuclear power plant can cause harm that will last longer than any human civilization has managed to last. The harm caused by launching Challenger is equal to something much smaller, such as a single day of car accidents.

There is also a false dichotomy presented here, because the alternative to nuclear is not limited to just fossil fuel. There are numerous forms of clean energy that doesn't have the failure possibility of nuclear.