Disclaimer: I am all for Nuclear Energy.
But the answer to your question is that it is not as clean as it seems, and after what happened in Fukushima/Chernobyl, no matter the precautions you take there are points of failure, and the cost to such failure spooks people out.
Why is it not as clean as it seems? Well, nuclear waste has to go somewhere, and due to the half life, it takes thousands of years to decay.
But out of all non renewable energy sources, IMO Nuclear Energy is needed, at least till we make the tech/infrastructure leap to other sustainable sources.
Hope Cold Fusion is possible, and someday that can at least mitigate some of the risk.
