The same cancerous lump that has every major publisher looking to expand their mobile offerings and chase that Candy Crush and Clash of Clans money, including Blizzard.
Mobile games are cheap as hell to create, so if they bomb or don't do well you lose very little. But if they do explode like a Clash of Clans, or even do moderately well? You're swimming in money at that point.
Like it not, the mobile gaming scene is huge and still growing, and just about everyone wants a piece of that pie.