Physics, specifically their much higher capacitance. The buried lines cost more, but mainly they aren't buried because of physics.

"A typical 345,000V transmission line will be able to deliver no power when the line becomes about 26 miles long."

https://www.puc.nh.gov/2008IceStorm/ST&E%20Presentations/NEI...

In New York City specifically most distribution (not transmission) is already underground.

http://www.nyc.gov/html/planyc2030/downloads/pdf/power_lines...