in the story line for PoR, they stated that the reason was that the Belmonts were forbidden to use the Vampire Killer for hundred years. Chances are Dracula cursed the bloodline or the whip. Eventualy Julius became the first belmont to use the whip again after that time period, and was the one who finnaly beat dracula.
Oh course this was done to explain all the Belmont knockoffs in all these game.
I'm still curious about Victor Belmonts story myself. Apparantly he renounced the Belmont name, and became a mercenary, but eventualy he was supposed to be dragged back into his destiny by some time warp. if The Dreamcast game had come out, maybe he might of been the reason the belmonts couldn't use the whip.