The fact that Blizzard didn't announce Diablo 4 during Blizzcon this past weekend caused quite an uproar but, according to a recent report, a teaser for the game was actually cut at the last minute.
According to Kotaku, a couple of sources familiar with Blizzard and their Blizzcon keynote have stated that an announcement for Diablo 4 was originally planned for the event, but eventually cut from the show. While a reason for this change of plans was not offered, Blizzard did not offer an official statement concerning the report.
Apparently, the plan was to have Blizzard co-founder Allen Adham walk out on stage and announce to the audience that Diablo 4 was in development, but not in a form the studio was ready to show at the moment. Rather than go that route, however, Blizzard opted to, instead, end their show with the announcement that Diablo Immortal was coming to mobile devices, which ended up landing with a resounding thud.
It's possible that Blizzard is really banking on the new mobile Diablo game and didn't want to steal any of its thunder with a reveal that didn't really "reveal" anything. They've admitted since the reveal that they expected some apprehension concerning the mobile game since it wasn't built for PC or console, but the degree to which the community responded negatively was unexpected.
It's also possible that the team simply didn't want to announce a game when they didn't have anything to show for it, but we've seen that even an extremely vague reveal can work wonders for amping up a community. When Metroid Prime 4 was announced, all Nintendo had to show for it was a logo. The same goes for Bethesda's Starfield, which I'd guess is even earlier in development than Diablo 4. But, in both of those cases, simply letting the community know that the game is indeed in development was enough to win the fans over.
Whether or not Blizzard should have at least given a nod to Diablo 4 during Blizzcon, the reaction from the community to Diablo Immortal was rather extreme. Folks asked some pretty rough questions during Q&A, with message boards and social media overflowing with angry messages from frustrated fans. Some have called out the game for appearing to be a reskin of similar mobile dungeon crawlers, though Blizzard has stated that this new game was built from the ground up as a Diablo title. There are also claims of the developer nickel and diming players with microtransactions and loot boxes and, while it's easy to understand concerns revolving around those types of game "features," no pricing model or plans for any type of microtransactions have actually been announced.
Either way, Blizzard has certainly taken some licks following what was planned to be a celebration of their games, so it'll be interesting to see how they respond to this kind of backlash.