Renewable energy at that scale is still theoretical and hasn't shown its downsides yet.
Nuclear has shown all its downsides, and they have been 95% corrected. It's a mature, known technology.
As for plant capacity, the largest power plant in the world is renewable (Hydropower):22.5 GW. Compare that with the largest nuclear plant, the Kashiwazaki Kariwa Nuclear Power Plant (currently closed due to safety concerns): 8GW.
If you limit "renewables" to only solar and wind, there are already 1GW+ wind and solar PV projects operating, and more in the planning and construction phase. There's no theoretical obstacle to building 8GW solar PV projects, though it might be unnecessary, as they can be deployed in a modular fashion. There are several 500MW solar thermal projects operating and more planned.
The downsides of nuclear are not comprehensively understood. The consequences of the Fukushima Daichi nuclear disaster of 2011 may be with us for decades or centuries. The interactions between radioactive material, the atmosphere, the ocean, the human body, and other organic matter, are complex.
Hydro works well and is the best solution if our country has capacity for it, unfortunately, it's not available everywhere.
> The downsides of nuclear are not comprehensively understood. The consequences of Fukushima may be with us for decades or centuries. The interactions between radioactive material, the atmosphere, the ocean, the human body, and other organic matter, are complex.
The downsides are completely understood and the consequences of Fukushima (please specify in your message the nuclear plant) are well studied, the result is that there's not much impact.
Hydropower is also an example of one of my points. It is a mature technology, and we now know of its big environmental costs as well as it's benefits. We could technically probably double the hydroelectric power of the world, but the natural devastation would be too large, so we mostly chose not to.
We have yet to see what those downside are for solar and wind. For nuclear we know, and also know how to handle them.
It's already hard to find a safe solution for waste that'll survive for millennia. Simply because predicting what might happen over those time scales is hard.
Now factor in that it'll be paid for by humans with a lifespan left of 50 years, and usually a decision horizon of 4 years (i.e. politicians).
The chances off getting it wrong technically are non zero. The chances of getting it wrong for budgetary reasons are even bigger. Incentives do not line up.