Because it takes 15 years to get a nuclear plant up and running. Which is plenty of time to sabotage the project so we end up stuck having to use fossil fuels.
Not to mention we would have to burn fossil fuels the whole 15 years we wait for that one nuclear plant.
I know this is kind of off topic but I wanted to point out that the refrigerant that escapes from air conditioners when they leak or are thrown away, is a bigger contributor to climate change than the electricity they use.