I assume it's a matter of universal scalability. There's a limited number of viable hydro projects, whereas solar and wind power is just a question of scaling up production generally. Most places are viable for at least one of the two.

I think its not built as much because solar/wind are simpler & cheaper. Hydro needs the right elevation/water flow/geography. Its disruptive to ecosystems & human habitation & it has huge up-front costs. Yes, its great sometimes, but maybe not as often as we might think.