Building reactors takes a long time. We probably first need to ramp up the industries that manufacture pressure vessels. The forging presses able to produce pressure vessels for modern reactors can manufacture maybe half a dozen a year, and there aren't too many of these massive presses around. You can of course argue that we build a different kind of reactor that doesn't require pressure vessels, but then you need additional time for R&D and proving the design in practice. Don't forget that we also need to train the engineers that staff the new power stations. Nuclear engineering is not a terribly popular major right now.
Wind and solar on the other hand are well suited to reducing our CO2 output starting today. Building a wind turbine doesn't take ten years.
