I think underground lines make some sense, I mean, think about it, how easy, or hard, would it be for falling trees to snap an underground line? Earthquakes and digging up damaged lines would be another story though.
Around here the water table is only a few feet down and would be a real problem.
Easy... they wrap their roots around them. Tree goes down, roots come up.. lines break.
Newer areas are under ground because even though the above can happen it does not happen as often.
Those overhead transmission lines will be cheaper. The ambient cooling of the air is verses building tunnels with cooling systems to string transmission lines.