Because from what I know, the fallout from it literally exploding is not making anything nearly that large "uninhabitable", even if we literally stand around and did nothing against it. Deaths would go up, but so did they from the coal plants that were subsequently built (we're at 3 million deaths a year, if memory serves; that's the number you have to beat to make nuclear less attractive than the technology we did build. Every year. Millions of people.).

Edit: a quick search suggests 4 million now, or another source says 10k/day which works out to 3.7 million per year (from results' summaries shown on DDG) but feel free to find reliable sources.