You know, I've often wondered that too but I guess I can understand to some extent. They want you to try the other characters. Also, the more I thought about it, the more I considered other multiplayer focused games like COD, which also has a level cap in its multiplayer. I think the same reason applies. They want you to move on to the next character. The great thing about Destiny is that with the RPG elements, an expansion can come along and increase the level cap.