Underground utilities have do deal with more intrusions from water, ice, salt, digging, backhoes, insects, rodents and other stuff you find at "ground level." Ask any outside plant engineer and they will tell you that cost goes up and reliability suffers when you place utilities underground.

Aerial utilities primarily only have to deal with trees and ice. Oh, and gunshots. In the backwoods, the local yokels will sometimes shoot at transmission towers, taking out the fiber links with them.