l think in the older games, the whip was the whip in its true form since it was normally depicted a frail in the opening of CV3. But I think once the graphics have improved like in Super CV, the whip is now leather.
It also was animated as a metal whip to show the gamer how better the weapon has become. I mean, I doubt in the cv mythos, alucard'z spear can level up like in the genesis cv game.
As others have already mentioned, the chain whip was probalby done to show how strong the whip have become. I mean, if Konami did leather whip-longer leather whip-flashing leather whip, it wouldnt be as impressive as goin from leather to chain.
With 3d graphics and the lost of upgrades, Konami has no reason to make it a frail unless it is the director's decision.