Why were nails pierced through the palms and not elsewhere?

Nails were not the typical method of attaching someone to a cross; they were normally tied with rope around the wrists and shins (nails cost a lot in those times; no organisation would use them for a petty criminal). Nails would presumably be put in the same places - if they were put into the palms they would slide out between the knuckle joints under the weight of a human body.