WalMart lobbies hard to make sure all their buildings have basically zero value as it reduced property taxes. They will put up the cheapest possible building and invest absolutely nothing into it. They deprecate it very quickly. Investing on solar panels on the roof or the parking lot just means more investment in a specific location that they don't care about.

As has been noted elsewhere, at present commercial solar PV can be 100% depreciated in the first year.

This is about to change, however.