For reasons of safety. Think about the sharply pointed Spitzer nose of one cartridge in a tubular magazine, resting in contact with the primer of the cartridge in front of it in the magazine. Upon firing, the recoil can cause the cartridges in the magazine to indent the primer of the cartridge ahead of it and to ignite that round. The results of such detonations are catastrophic, frequently resulting in the destruction of the rifle and serious injury to the shooter. Do not use Spitzer bullets in tubular magazines!