Most people have experienced the sensation, perhaps when on holiday, when feet are released from the confines of shoes, and for some time are very sensitive when walking barefoot, but after a while, become less and less sensitized, as the skin and nerve endings become habituated to it.
The bottom, on the other hand, is almost always well covered and is not usually in contact with anything more sensorally stimulating than a hard seat.
For this reason, I believe that the buttocks tend to be more sensitive to sensations, especially powerful pain-producing stimuli than the hands would be.