The additional roof load wouldn't have been calculated in the initial structure, and thus it wouldn't be safe to put it up.
Furthermore, whether it would have been cheaper initially to build the roof in such a way as to be a solar farm, or buy extra land is not as cut and dry as you would think.
Every successive beefing up of the roof adds to the building cost disproportionately, and you still have to deal with the fact that your workers and maintenance guys are all on the roof, every fastener needs to be water tight, you need to be so careful of the membrane, etc etc etc.
Even if they did plan on the solar right from the start, there is a good chance, depending on land value, that it would still be more economical to do the solar install at another site.
Besides, the less 3rd party workers in and around their data centers, the better they feel. Physical security will be immensely important to them.
Karl P
Edit: Fix Clarity.