Main reason is cost. It's incredibly more expensive to bury lines than to run them on a pole, even taking into account the *chance* of outages like this and the associated cost of the outage (versus the *certain* cost of burying them).
Also, things like tree roots, etc. can cause problems with underground lines, and outages can be more difficult (and time-consuming) to correct when they do happen.
Many power companies will gladly bury lines if someone (i.e. a city entity, neighborhood association, etc.) will cover the extra cost, but many are not willing or able to do that.