Cost, difficulty, and harder to maintaince. Want to add new coax cable? Sure toss it up. New housing being? Split it. Fiber, yep throw where the coax is. Etc. (its still high voltage, so regulations for safety obviaouly play a role here too).

Underground? Is there rock too hard to drill there? A gas line? Did we just cut an active internet line or just some junk? Tree roots? Will it containate a water table? Will it shift and break the line here? Great we have conduit, is it broken? Leakage? Big enough to handle a high gauge for an upgrade?

i say this as man in love with a good tunnel and conduit. Trust me when i say, yes we ought to do it in general, but also yes is a pain in the ass.