The thing is as big as some MOBA's are they carry a stigma with them of being too "noob-unfriendly". I imagine Blizzard All-Stars will be Blizzard's take on the genre focusing primarily on delivering "accessibility" to it's players along with a selection of heroes from their pantheon of characters. And don't forget about that Blizzard marketing budget. Somebody already playing Dota 2 or LoL probably won't give All-Stars a second look, but I imagine it will still find a healthy audience.