Because we have limited labor/money to spend on these things and rooftop solar is a lot more pricy per unit of power it produces.

Also, I personally don’t like the tension between shade trees and solar that OP is referring to here. It causes a lot of people to cut down or not plant trees—trees that will be essential in keeping people alive during the extreme heat waves of the future.

I also think we should move towards using roof space in functional ways—either social or gardening space, at least in areas that don’t have excessive amounts of snow that make peaked roofs necessary.

So, I’m not saying rooftop solar is evil. It can be better than the status quo, and I’m actually thinking of getting it myself. But it is a kind of hyper-individualist solution that I think would only have a small role in my ideal solarpunk society.