It's the same problem PR faces with the medic class: It's not enough for a "fighter class", so people don't spawn as it. Which is sad, as it's a useful class that has lots of potential and is also important for teamwork. Hopefully players will start to realize this.
There's also the problem that mines attract idiots. It's simply amazing how, when you mine a passage, it suddenly becomes all-important for at least two players to run through it - people who for several minutes were completely uninterested in it. I've found that mines kill ten times more friendlies than enemies - at least - which means they're in essence utterly worthless.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.