Because the reputation system is based on the social structure of the game, meaning it has much more to do with NPCs, simulated society, geographic location, jobs, etc. The Karma system, on the other hand, is a universal system which should be applied at all places equally, and it\'s soul purpose is to manage pvp outside of controlled areas. Similar means, different ends. Also, karma would probably be far less permanent than reputation (not meaning it\'ll be quick to change, it\'ll probably take a good couple of real life hours to regain the karma you lost by killing one person).
As for the idea of areas where karma doesn\'t apply: I only feel this should be true in places where no player is allowed to kill another player no matter what. Such areas would probably be needed even if the karma system was installed. I disagree with the idea of \'evil places\' because the karma solution is geared for open-ended game style which probably should stay, for the most part, constant, even in wickedly evil areas. Such areas could do just as well by having extra nasty monsters. But then, I couldn\'t know unless it was tried, it could end up working well.
Something tells me that players who play assassins will be given simulated quests killing NPCs much more often than playing actual players, so that\'s probably not much of a problem. When assassins do kill other players then it probably won\'t be often enough to warrent a really low karma, at least not below 10. People can still probably kill one or two players a day while still keeping it managable. The karma system is set up to make sure people don\'t abuse pvp, like killing 10 people a day, which sounds like a really busy assassin to me.
DizzleCorinthos: Actually I think it makes more sense to rank karma by numbers and reputation by titles, seeing how karma is more of an actual stat and reputation just seems more like political jargon (not implying I don\'t like the reputation idea; I do).