One of the main reasons is the nuclear waste which for the purposes of the timescales of human civilization and planning are indefinitely dangerous. In the US i

One of the main reasons is the nuclear waste which for the purposes of the timescales of human civilization and planning are indefinitely dangerous. In the US in particular there's never been a great plan implemented to actually deal with that waste long term. It mostly just gets shuffled around to current running reactors when one shuts down.