Because it's irrelevant in dungeons. Yes, you can save some time and I do use them, but it doesn't matter if no one uses them. Frankly, it's not fun to pause your rotation to cast a DPS LB, especially if it's not LB3. Also, ranged LB is also not as satisfying and a bit trickier to use.
And with no LB3 in dungeon, healer LB is also mostly irrelevant other than for those (I hope) rare moments where the healer runs out of MP or something and really needs the heal.
I mean, if the tank/healer wants to use an LB in dungeons, I don't mind.
