Because that's how it was in 1st and 2nd edition AD&D, and I'm used to it. Hard styles did more damage than hard/soft styles, which in turn did more damage than soft and vital styles. Plus, it prevents martial arts from being "cookie cutter" identical to one another. If ninjutsu deals damage as a dagger, kungfu deals damage as a club, and karate deals damage as a heavy mace, there are suddenly different reasons to use different styles, and that's a good thing.