I think it's more that there's absolutely no content announced that's playable on a PC.
Think about the kind of people who spend X amount of money to attend Blizzcon. Some travel there from the other side of the world. It ain't cheap. These are the most die-hard Blizzard fans, so it stands to reason that they're all going to be PC gamers.
So Blizzard announced a new Diablo game that is only going to be playable on a mobile device.
Maybe a few people at Blizzcon do play mobile games alongside their PC games, but it's not going to be their primary gaming device or preference.
Diablo has (traditionally) been a PC game franchise. Sure you could get D1 on the PlayStation and D3 is available on consoles, but mostly it's a PC game franchise.
The mobile game probably would not be receiving this kind of hate or backlash if it had been announced alongside something else, such as... well, anything playable on a PC. More D3 content, D2 remastered, any kind of confirmation of D4 being in development.
So yeah, to announce the mobile game to an audience composed almost exclusively of PC gamers ...I'm not sure what else Blizzard were expecting as a reaction. Maybe this will be a big hit for Asian markets like China, but not in the West.
I really hope they've got something else in store for Diablo but based on what we saw yesterday I'm managing my expectations for now. Can always play more Grim Dawn I guess!