Trenching (dirt/grass) and boring (road or driveway) are both much more expensive in comparison to just running it overhead with poles. You first have to add the surrounding conduit cost in to properly protect the cable from water damage compared to just hanging it up. Easements are about the same I believe between the two but I'm sure, depending on the place, those underground easements will have to deal with more obstacles such as existing water and waste rather than just throwing up a pole every 200'.
Then there is also the troubleshooting issue for line crews for if a cable faults out, you then have to dig out a much larger section to first find the issue then to splice it rather than just splicing the overhead line with a normal line crew and splice kit.
I can't remember the exact numbers but I think the ratio for overhead, trenching, and boring was around $4/6/8 per foot making it over twice as expensive to bore over just throwing up some poles. Numbers can definitely vary, but most utilities probably won't want to trench and bore unless they have to.
Solar is not the solution for Alaska's load. Alaska is a solidly winter peaking usage and a very noticable dropoff in the summer. Wintertime, when there is the most usage, we have about 5 hours of daylight.
In the summer, we have plenty of daylight and very little electrical usage in comparison. Most people do not have ACs up here so unless electric cars and ACs start becoming much more popular, solar has the chance to do more harm than good to the grid during the summer.
I am all for renewables but solar is not the renewable Alaska should be focusing on.