Comment by sparky_z

The explanation I was once given (by an elderly female librarian at a bar, long story) is that all the nerves in the clitoris are just there to provide the same pro-mating pleasure signals as all the nerves in the male penis. But if they were spread throughout the entire ring of the vagina, (as they are throughout the penis) they would be extra painful during childbirth when the whole opening is stretched and torn. Extra nerves mean more exquisite pain signals (think paper cuts on the fingertips vs elsewhere.) It might even damage them long-term. Concentrating all the most sensitive nerves in a tiny spot at the top makes them available for pleasure during sex but minimizes damage and pain during childbirth.