Resources, licensing, gimic, patents... The list could be very long.
However, there are more developers adding more destructable environments to their games, however, I believe most of them are using the Havok physics engine.
I don't know... the premise underhands the whole deal of find x key go to y door. You just blow the walls out... however, how it turned out, you could blow up anything that wasn't important. You could supposedly drop sections of wall on people too... but I remember spending 10-20 minutes trying to carve out a sizeable chunk in the testing map. It is fun, just not really practical. Also, it can cause more trouble than its worth. What if you trap yourself or alter the map to a point where the scripts wont work or use up so many resources the game just crashes.... theres a lot of questions and reasons...