My theory is that because it's a very locked-down and incredibly competitive game, most people don't actually play 'Vanilla' SC2 for long, so Blizzard added moddability to SC2 to keep people's interest up between expansions... and maybe/probably as a test platform for a long-game strategy of monetizing UGC.Quote from MakaIf anything, the extremely competitive environment of SC2 would disqualify it from UGC before any other game I can think of, for fear of disturbing that competitive balance.
The fact that Blizzard allows it for SC2 and not D3 (a game they themselves admit is, first and foremost, a coop experience) is very confusing to me.
There's also the fact that SC2 games are discrete but somewhat broader in scope, and lend themselves well to all sorts of bite-sized experiences. D3 OTOH (and RoS much more so) is more open-ended in duration but with a fairly tightly-defined user experience. I'm not sure I can imagine a D3 Bejewelled or Tower Defense mod... I can sure as hell imagine a WH40K mod though, but that thought makes me feel funny in my pants.