It's because any modern RE technology requires hundreds times more land than gas or nuclear (surface power density) so if you had a nuclear power plant that occupied say 1 km², replacing it by wind or PV means you suddenly need to occupy area of 70 km² to get the same nominal power.
There was recently a dispute in Kent precisely on that topic - is it really so "green" to occupy 400 ha of forest, moorlands and meadows with PV panels...