It's just the fact that renewables aren't enough (since it's not always windy or sunny) and storage just isn't cheap or plentiful enough. They're great in many ways, but they alone won't provide enough power and stability.

So you're left with water power (which is only applicable in few areas and they destroy the nature), coal/oil/gas (which are much worse for the environment) and nuclear energy.

Nuclear might not solve everything itself either, but it's definitely part of the solution.