Underground is harder, you have to dig up land every time you want to fix or replace something, this can be mitigated but it still isn't that great. More importantly in urban areas digging up land is really hard (sidewalks, roads, residences, buildings, water pipes, sewage systems...), not to mention the politics!

What is there to fix or replace? A properly insulated transmission line should last indefinitely.

Stringing taut wires is also hard and they are more susceptible to damage which require constant fixing (see Connecticut ~4 years ago).