Because clean is only if you don't include the waste and you never have a disaster. Literally the only way nuclear is clean is in the best possible case scenario, which happens for the most part because of the training and safety that are in place in most countries, but when it doesn't it's bad and impacts huge chunks of the planet. We have had nearly 100 accidents, most occurring in the United States according to Wikipedia. That's not a small number of accidents considering how long nuclear power has been around, it's more than one a year with the majority within the last 40. Most aren't major events, but I'm not sure how minor you can really claim a nuclear event to be. This isn't too say the fossil fuel alternatives are better, but nuclear boosters aren't really doing themselves any favors by glossing over the negatives.