Because lots of players come over from Halo or other FPS games to play PvP and expect everything to just be available at the beginning, like it is with FPS games. Outside of weapons upgrades, which are simply handed out in PvP matches in those games, there is nothing to upgrade: no new skills, no character progression, etc. It's that mindset that brought us arena and welfare epics, and it really pisses off the rest of us who cut our teeth on 60+ hour RPG epics and have been playing RPG's and MMO's all our gaming life. We enjoy the progression, the time investment, and the feeling of accomplishment when that piece we've been looking for finally drops.

When these FPSers come over and whine and whine until Blizzard gives them what they want, it sticks in our craw, and we would prefer that they just go back to Halo and stop messing with our game. The next time you get ganked and have your corpse teabagged, you can thank Halo and the other FPS games, because you can be sure that we didn't come up with that particular nicety.