Because that's the transition power that has the math and history to back it. Its window is closing though. It would have made a ton of sense in the 80s or 90s but the antinuclear have won: we stayed 40 more years in the fuel economy until renewables matured enough to be a credible alternative.

We are not there yet though: intermittence (and to some extent construction speed) still favor nuclear power but maybe not for that long.