Because it WAS just bad marketing.
I may be mad at this game myself, but I am also a marketer, and I have already seen this year two other major franchises get away with this simply by following up the mobile game announcement with a title card for the "real game" that won't actually see a store shelf for the better part of a decade.
Consumers don't actually care if Diablo Immortal exists or not they only care that they don't feel abandoned by a franchise they really like. And announcing this game as THE big announcement of Blizzcon without a breath of encouragement that Diablo 4 was coming is precisely why this is news at all.

Diablo Immortal is a game made in China for a Chinese market, and will make Blizzard a fuckload of money from that market, but announcing it here at Blizzcon acting like it we were suppose to be excited for it, is one of the most out of touch PR blunders I have seen in a while.

Problem is they will think of this as bad markerting instead of they did a bad thing. They won't learn anything and nothing will change.