Wind doesn't blow all the time. And wind isn't able to throttle up to meet increased load (unless you massively overbuild wind farms... then subsidize the heck out of them; several orders of magnitude more than what the USGov currently does for fossil fuels). There are ways around this (ex. pumping water uphill, then flowing it downwards through an impeller connected to a generator... or you could use batteries if you want lower efficiencies), but they have significant costs associated with doing that. Also, wind tends to blow more consistently at night, when demand is typically lowest; you don't need as much power for AC at night as you do during the day (It's windier during the day, but the wind tends to shift around more, making it less useful for power generation). Also, because of the loading variability, wind turbines tend to have extremely high maintenance requirements. Plus, the windiest spots (where the power is generated) aren't typically close to where it's used.
Hydro could do it all, but there are almost always heavy political costs for doing hydro, even compared to nuclear; and you can only do it near a river that isn't too heavily populated. Geothermal only works well in a few places on earth (ex. Iceland). Solar typically isn't very efficient, at least from a bang for the buck perspective, and panels aren't very environmentally friendly to make. Future improvements in solar towers could change this, and more research should be done into solar driven Stirling engines; but neither is there yet.
Which leaves a need to include something with more flexibility into the power mix. Right now, it's either fossil fuels (oil, coal, NG) or nuclear. Between the two, nuclear is cleaner (taking into account the nuclear waste issue, production cycles and mining related issues). Nuclear could be much, much cleaner if reprocessing were taken more seriously.