Diablo 3 plays differently on the PC fro how it does on console systems. With the keyboard and mouse, the game's pace differs from that of its console counterpart, which makes full use of each of the controller's buttons and draws the player's focus onto the character itself and not where the mouse cursor (which isn't there) would be.
Blizzard's Matthew Berger explained the rhythm of the gameplay on PC, and why the PC won't be getting controller support.
“The rhythm on PC is tactactactactactac – you’re clicking the mouse, you’re telling your character where to go, the rhythm is very different," Berger told Eurogamer. "It flows more, because you’re constantly repositioning your character with the thumbstick; you’re really never stationary on console, whereas on PC you’re a lot more stationary."
“When you’re playing on the PC, you’re not really looking at your character as much, you’re kind of focusing on the cursor. Whereas on console it’s the exact opposite, you’re drawn in.”
“If I let you use the controller on the PC, then I have to let you use the user interface that goes with it, and if I give you the roll and I haven’t throttled the number of enemies attacking you because the mouse-and-keyboard players can handle all those enemies, but with a controller you have too many guys,” Berger added.
“The games have really been structured to take advantage of their environment and their ecosystem, so in the same way it would not be a good fit to put a mouse and keyboard on the console. It wouldn’t work.”