The inside of the bolt carrier where the recoils spring goes is not all the same inside diameter. Just past the nose where the piston threads are, the diameter gets bigger and it can't be tapped for the same thread size. If I had just cut back the carrier and went to a bigger thread size, the nose of the carrier would lose some of its taper. That might have effected the reliability of the carrier to feed back through the front sight block and gas tube.
Besides.... I like to do things the hard way.