Here's a great video about an LA underground power line. It goes over many of the downsides of underground power lines.

TL;DR

1. The lines need to be insulated from each other. Above ground, they can just be hung far away from each other. Under ground, you need to spend more money either building a large tunnel or adding insulation.
2. Locating where underground lines have been damaged is a pain in the ass (more expensive and longer down time).
3. Digging up the lines to do the repairs is also a pain in the ass (again more expensive and longer down time).
4. Just doing maintenance is a pain in the ass. So underground lines won't have trees falling on them, but they'll just fail because it's not worth doing preventative repairs and maintenance.