Nuclear is too slow. Not only are you running fossil plants for decades while you build nuclear reactors, you are also emitting additional CO2 during the construction process!
Renewables reduce CO2 emissions today. Building a solar farm takes months, building a wind farm takes single-digit years. They'll have recouped their construction emissions well before the first nuclear reactor comes online.
It is important to remember that the challenge we are facing is not *how do we build a fully-renewable grid", but "how do we reduce CO2 emissions the fastest". Having a 95% solar-and-wind grid with 5% fossil backup 10 years from now is significantly better than having a 100% nuclear grid 20 years from now!
Building enough nuclear to replace fossil electricity generation is both insanely expensive and not enough to fix the problem because plenty of emissions don't come from electricity generation.
