1. Because Blizzard made it. To me blizzard wasn't mortal until Diablo 3 came out. Up until then it didn't bother if me they delayed games for months and months and years and years because I knew the game at the end would be nothing short of godlike.
2. Diablo 2
While I wouldn't exaggerate to the extent of this post, in general, this is how I felt. I also was eager since we had been waiting well over ten years for a new Diablo title.
Had they not titled this Diablo 3, I would've possibly still tried out the game, but if I had, I would have enjoyed it a lot more. Giving it the title "Diablo" set certain expectations. Unless their stockholders were looking for a huge short-term gain, they did a disservice to them by naming this game Diablo 3. Certainly, in the short term, RMAH, AH and calling this game Diablo 3 were brilliant business moves. But, in the long term, it has divided the Diablo gaming community, heck a lot of the Blizzard community. This reflects poorly on the company and might lose them subscriptions and stockholders.
Plus, now who will buy the expansion other than die-hard fans like me (even I might not)? If the expansion is more of the same nonsense (which is an adequate, not great, not enough content game) then Blizzard won't fool consumers as much this time around as they did with Diablo 3 and Diablo 4 sales (assuming there ever will be a Diablo 4) will be even worse. This is how Blizzard has done a disservice to their fans and stockholders.