It is because to make meaningful electrical generation, they require large surface areas, and easy access for cleaning and maintenance. To replace a traditional electric generation plant, it would require square miles of coverage. Additionally, solar panels are only 30% efficient, when all in direct sunlight, which I agree is better than nothing. However, if any part of solar panels are shaded by clouds, the efficiency of the whole array is reduced. Load forecasting is difficult because the electricity generated is use or lose, and there is no storage.
Legoland has covered solar parking, and helps, but to solve our increasing electrical energy needs, solar is not enough.