According to sources close to Kotaku, Blizzcon didn't go exactly as planned for the Diablo team. Allegedly, they were going to announce that they were working on Diablo 4, but with no concrete vision in place, they pulled the announcement - hence the news post from the official Blizzard/Diablo site:
"One of those people told me that the Diablo team wasn’t yet ready to commit to an announcement, as Diablo 4 has changed drastically over the past four years and may continue to change further. (We’ve heard it’s gone through at least two different iterations under different directors.)" -- Jason Schreier, Kotaku
Blizzard has yet to comment on the story, so we cannot confirm whether the sources are correct. Although, if you do read the news post again, they do mention that they aren't ready to announce all of their plans at Blizzcon. Was this it? Hopefully we'll find out soon.
Kotaku has updated their original article with a correction: Blizzard did NOT plan on announcing Diablo 4 at Blizzcon this past weekend. Here's the statement that they received directly from Blizzard:
“First off we want to mention that we definitely hear our community. 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. We do continue to have different teams working on multiple unannounced Diablo projects, and we look forward to announcing when the time is right.”
Their sources still confirm that they are working on Diablo 4, but that it still has a long road ahead.