That's the only reason it was so popular, it was the only authorized expansion pack. Back in the day, Vivendi Universal Interactive used to own Blizzard and Sierra. Sierra created Hellfire and Blizzard authorized it. It was the first and last time Blizzard worked with Sierra on a game, and allowed any other company to create an authorized expansion to the game because of how bad it was, :P.
Well, not that it was bad, but it contained bugs and was missing a lot of features that were supposed to be implemented, but did not make the release date.
Anyway, if you still don't believe me, check out the CD case. Top-left corner says, "The only authorized DIABLO Expansion Pack". That was the clearest pic I could find on google. I'll upload a pic of my CD if you need more proof, :P.