Many of the easements for above ground utilities also contain provisions for underground utilities, so getting easements may be easier than you would think. However, having the easement, and having it be feasible to use are two different things. In my neighborhood, some sections have the utilities in line with the sidewalks and there is easy access; others have the utilities behind the homes, over many years many buildings have been built encroaching the easement. The encroaching buildings aren't legally there, but it would be messy to remove them to underground the utilities.

Also maintenance and access. I once had an underground power line to my apartment building fail, and it took the better part of the day for them to repair it. They had to tear up the parking lot in the process.