Being on the poles can be more expensive than underground. Trees, cars and even bullets can damage the wiring. Underground is more expensive to start with but cheaper long term.
Underground has a bunch of other issues as well, in the UK all records of the underground ducting have been lost in many areas in the 50-70 years since it was installed, which makes it a big headache.
Which makes fixing, replacing etc quite expensive and the quality can be suspect.