Idk I think people just overhype D2 and what it actually was. I myself probably played something like ~1000 hours of D2 in my life.
(Some people on here sound like they spent more than ten times that. I have to question your definition of "fun" then.)
It was something like:
~200hrs when it came out
~200hrs when the expansion came out
~50hrs every year.
On and off, always with friends, sometimes after a ladder reset.
It was a solid LAN title and a fun game to pick up now and then but nothing I would play continuously (for example like MMOs which usually net 10hr+/week).
I can also tell you why I didn't play more D2. The game has extremely little content. I'm pretty sure I could rush the original D2 Hell (probably even with the expansion) on my own in way less than 10 hours on most classes (softcore of course).
Endgame D2 is EXTREMELY repetitive: Teleport to Andy/Meph/Baal => Portal for group => Kill => Loot => Restart.
So yea I played D3 when I came out for ~200hrs. Beat inferno on all classes (softcore). Came close to beatin inferno on hardcore on my wizard (in 1.0.3 if that makes a difference to anyone). And then I quit. 50$ well spent (25c/hour). I will defniately get the expansion too when it comes out.
Failed? Not in my book. Completely what I expected.