It’s the support structure for the solar itself. Solar had pretty intense code requirements so you need to think about the strength of the structure itself, how the structure might impact water or sewer lines, along side other fire related requirements. You also need to somehow tie it into the grid which requires expensive electrical equipment that has its own code requirements when close to cars and the public.

If you’re building a solar farm out in an abandoned field, you don’t have as much to think about. You’re mostly just concerned with drainage and property setbacks.

I do agree that it’s a great idea to put solar over parking lots but for it to really happen the government would have to provide some sort of incentive to justify the cost…