One of the main factors chrisgel is, out of sight out of mind. And lets be fair, its not an easy subject to take on, not only because of religio-socio-cultural elements, but by the very nature of it being young girls bodies.
The other fact is that the numbers reflect 'British' subjects, so these people will take their girls abroad to inflict the horror on them, and unless the child is examined on return, nobody would know, and as per above young girls are either going to be brainwashed, or just unlikely to verbally reveal to anyone what has gone on down below due to personal sensitivity issues.