The venue is just weird, why would they even announce a Diablo mobile game at BlizzCon? The people who care enough about Blizzards games to attend or follow BlizzCon DO NOT CARE about mobile games, at all.
The people handling marketing and products at Blizzard, and EA, and Bethesda aren't in touch with their most loyal fans. I can only imagine how much money there must be in in-game transactions, given that all three companies continue to upset fans that don't mind paying $60 for a new release.