Ten things are probably easier to remember than twenty. But thats ok, its only the level system, you dont need to know it. Also there is a dilutative aspect to having more levels. The importance of each level declines to a certain degree. This makes levels more fun and hopefully less distracting. Hell if we had 50 levels then nobody would care one whit which level you were, beyond that the levels may or may not have cool names. Part of the intent of this is to reduce focus in the XP/Rep/Level process. Having a zillion levels would clearly facilitate this, but we have to balance against other requirements. 20 odd seems about right, give or take a few levels.