It's a combination of things, really. Underground is about twice as expensive as above-ground, harder to maintain and repair, and pretty much impossible to upgr

It's a combination of things, really. Underground is about twice as expensive as above-ground, harder to maintain and repair, and pretty much impossible to upgrade. A lot of the medium-voltage stuff is already underground in some areas. All new 50kV-150kV stuff has been going underground for decades, and the stuff remaining above-ground just hasn't reached its replacement age yet. Despite its drawbacks, above-ground is unpopular enough that burying them is worth it. However, the high-voltage stuff (220kV & 380kV) is still almost entirely above-ground. Those cables are extremely difficult to construct, and anything beyond a dozen miles or so simply isn't technically possible yet.