Actually it probably is.
Warlords was not good, despite what people will tell you about how what it HAD was good it overall lost players and is strongly criticized for various good reasons. Legion was by all rights a good expansion, but BFA and SL were both abyssmal expansions that sucked as much if not more than WoD.
Broadly speaking 3 out of the last 4 expansions have not held up to scrutiny and for a lot of players that really broke the desire to hop on the NEXT EXPANSION FIXES IT train or the DONT JUDGE IT TILL WE SEE IT boat.
I still don’t have DF and may only actually finally buy it next time it goes on sale. I’ve heard good things about it, but ultimately all the previous expansions launched in decent enough states to the point where cracks didn’t start to show until later, and then Blizzard would fix them so I refused to jump back into it just because initial launch felt good.