Because there's so much shit to master. Offticking, speed runs, no pillar runs, and the list goes on forever. It's fucking great. How could the same braindead and repetitive "spamming ice barrage on another prayer and swap gear be fun? It's something you avoid doing because of how pointless and retarded it is. The Inferno, on the other hand, actually has a skill cap and there's something to learn.
Then don't expect to receive the rewards only attainable through PvM.