According to Blizzard, “However, as everything started coming together and we’ve been listening to and building upon your feedback, it’s become clear we need a little more time for additional polish, and to balance and iterate on some interlocking pieces—particularly those related to the endgame.”
The company cited that the team having to work from home played a role as to why the expansion has been delayed. There was actually some speculation about the expansion being delayed ahead of the announcement. This is because the game was supposed to be released on the 26th of October, but the pre-patch had yet to be launched.
Blizzard typically gives gamers about a month with the pre-patch before the expansion is released, so the fact that the pre-patch is still MIA, it led to speculation that there could be a delay. That being said, the pre-patch has now been given a release date for the 13th of October, so hopefully we’ll be able to get our hands on the expansion in November.