There are probably lots of different reasons. But they will all boil down to whether it really made financial sense in the long run.

In my area there’s a lot of rock. So burying lines can have a much higher expense even in the long run.

And I’ve also lived in some areas with a lot of hills/mountains. Burying lines in areas like that means you need a lot more line than if you just stretched it between poles or towers over the valleys.