Apparently Immortal wasn't to be the only big Diablo news and was simply meant to compliment the announcement everyone came to see.
Meanwhile, Blizzard's sister company, video game publisher Activision, timed a free giveaway of sci-fi shooter "Destiny 2" on PC to coincide with Blizzcon.
Following Blizzard's Diablo: Immortal announcement from BlizzCon 2018, which led to an immediate negative response by fans, Kotaku reports that Blizzard's original plan was to release a tease for Diablo 4 at the end of their announcement.
If Blizzard had shared a teaser for Diablo 4 at BlizzCon, even a wafer-thin one like a GIF of a flaming "4", Diablo Immortal's reception may have been more positive. We generally don't comment on rumors or speculation, but we can say that we didn't pull any announcements from BlizzCon this year or have plans for other announcements.
As for the reason for the delayed reveal, they seem to be perfectly reasonable: the design of the game has changed multiple times over the last few years and Blizzard still aren't sure they've locked things down enough to announce it. The announcement was set to take place with Blizzard co-founder Allan Adham revealing that Diablo 4 was in the works, but not ready to be shown yet. The publisher also confirmed that multiple teams are working on Diablo projects, and they will be announced at the right time. Blizzard maintains that it had never meant to talk about Diablo 4 at BlizzCon and that no announcements were pulled from the show. Diablo IV's development has probably been ongoing in some form for the last six years or so after Diablo III had shipped.
Blizzard has been attempting damage control since the Diablo Immortal announcement hit the web, so at this point, it is hard to lean one way or the other.
Fortunately, game development isn't like dating, because if it were Blizzard might consider calling the whole thing off.
Morgan Stanley's Brian Nowak suggested "Diablo Immortal" could eventually have more than 200 million monthly active users, and that it could generate annual earnings of as much as $2.52 a share.