Short buildings don't have a lot of windows to make it worth while and are likely surrounded by trees, bushes, and other things that case shade. Tall buildings have lots of windows, but tend to be surrounded by other tall buildings casting shade.

Roofs are flat and easy to work on and are more likely to have sun for more years and more of each day.

If the building is being built and there is money in the budget for "solar windows" even if they don't generate much power, the money would be better spent better insulating the building to lower heating/cooling costs or investing in more efficient heating/cooling.

Saving electricity on more efficient heating and air conditioning has exactly the same effect as generating more power using "solar windows". In fact, you might save more power because you can only generate electric from about 10am to about 4pm, but heating/cooling runs outside those hours.