Because nuclear requires enormous initial investment, it's slow to deploy, tricky to distribute, not easily scalable, still needs to be accumulated (because of too much power, rather than too little), still relies on the economics of a fuel.

If we invest heavily in nuclear what we could invest on renewables, we would reap the profits in twenty years.

And we can't make small reactors; for one, because tech isn't ready yet (despite recent Chinese investments), but also because... Who'd want them in their yards?

Also, since you mentioned it... You'd be amazed how little we progressed in renewable tech in the last 20 years, especially solar... We really sat on our comfy gas for a long time, you know... No real incentives...