Because it was designed for the round. I don't shoot .38 spl. in a .357 and I don't shoot .44 spl. in a .44 mag.. Ever since I drove 70 miles to shoot and couldn' chamber a magnum round because of shooting the shorter round, I use the proper cartridge in the firearm it was designed for. I also believe that the special rounds (.44 spl. & .38 spl.) actually shoot slightly better in revolvers chambered for the round.

Good shooting,
Steve