This has been pretty extensively discussed in a few internal ISP-specific forums... Basically, because its longevity will be limited. Snowplows and other things will mess it up. Maybe it's useful if you want to go across the back edge of a parking lot between two buildings where it will never see real vehicle traffic. Microtrenching is a valid alternative.
Aerial fiber is pretty much always preferred as it can be 1/20th the build cost of underground.