Reduced fire risk is the main reason. The $ value of damage after a big fire often isn't THAT much smaller than the cost of undergrounding cables in that area. There's also some fringe benefits for aesthetics, electrocution risk, and maintenance costs (underground maintenance is more expensive but less common)

Power on telephone poles have a couple problems, mainly involving trees/cars/miscreants breaking them and causing power outages (and sometimes fires). Some people also feel they're unsightly.

However, underground cables can be also be broken (e.g: by water leaks or backhoes), and in these cases, it can be more expensive and time-consuming to find and fix the break. I believe initial installation is also more costly.

Prevent fires - and overhead power lines are ugly AF. You don't notice it until you go to a place where they don't have them and wonder why the scenery looks so nice.

They're much safer, have much less maintenance, and higher lifetime 'uptimes'.