Because Diablo 2 is 7 years old. If you compare TH to D2, as if they were games in the same genre, you'd expect TH to be superior in almost all ways. When it's not, it doesn't reflect well on the game.
And... it's not superior in all ways. I'm not a dungeon hack fan, but apparently coop is a big deal for people who enjoy the genre, and Diablo 2 did that bit really well, while TH's coop is 2-person only and doesn't allow you to really play the story in tandem. Diablo 2 is also apparently longer; you can play through in 10-15 hours, I hear, if you've done it before, but a first run-through will usually take quite a bit longer. TH is apparently 10-15 hours long. I might be wrong on this last one; I'm going on anecdotes, both from the internet and from people I know.
So... comparing TH to Diablo 2 does it no favors. If it's better than Diablo 2, it's 'expected' since they had 7 years to refine on an existing game. If it's worse, it's much more shameful.