From Diablo Wiki
Blizzard has repeatedly stressed that there will be no guns in Diablo 3. Their reasoning is that guns are scientific and modern and don't fit in the dark, gothic, fantasy world setting of the game.
That the Demon Hunter's dual wielded Hand Crossbows are essentially machine guns makes clear that their "no guns" decision is about style and story and lore, rather than anything functional.
Blizzard Comments on Guns
Fans bring up this issue from time to time, pointing out that other fantasy RPGs (including World of Warcraft) have guns, and that the cannon technology isn't radically different or more advanced than some other things in the games. The developers and their spokesmen always disagree. For example, here's Bashiok in January 2010. 
- Guns are decidedly scientific, and the Diablo world is decidedly magic (with a lot of impaling people on pikes). Not that guns and magic can’t mix, but when they do it becomes something entirely different. So I don’t think it’s an issue of guns not being fun or just not working, it’s nothing like that. It’s simply a setting and tone issue, and guns would very certainly change it.