A big driver is that wind and solar have only recently become economical. The price of solar power has fallen by 80% since 2010, so large scale plants are all very new. Capacity factors for wind are also improving rapidly, from 25% in 2000 to over 40% today.

Basically we've had 150 years to build out hydro, 70 to build out nuclear, and only a few for wind and solar.