The reason is obvious. That is why militant sanctuaries have multiplied.
Just like what is happening in the latest episode in Bajaur. FC sent 200 men to capture the LoiSam post where the Taliban were able to surround them in much larger numbers. FC had to retreat and the Taliban have captured almost all of Bajaur and have now surrounded Khar, the headquarter town. Now, the FC is not short of men. Why couldn't they send a much larger number of men, knowing well (or atleast should know well) what the numbers of Taliban are? Now they have surrended the ground to Taliban and retreated to Khar and using bombing by planes and helicopters. This should be eye opening for the "fighting own people" mantra when your own forces can't move freely or go into "our own sovereign area".