Because the game wasn't made with a controller in mind. I can't begin to stress how difficult it would be to play with one. Just getting to a skill on the hotbar alone would be an exercise in frustration. By the time you have reached skill 6 with a controller you'll have wasted probably 3-4 global cooldowns, maybe more.
If they altered the UI a bit, they could do something like DCUO. Left shoulder buttons could be shift+ctrl modifiers while a right shoulder button changes 1-4 to 5-8.
Most people don't use anything beyond 1-6 since it gets too far to reach on the keyboard.
The only issue is all the hotkeys for maps, journals, etc.