Er... the book expressly describes how Szass Tam has sent a party of his own to investigate and shut down or claim the Soulmonger. It's one of the first things that the book talks about, actually! It matters since the players are likely to meet them, I think, but I've not gotten past the first section.
And the players will succeed where others have failed for the same reason that Indiana Jones always manages to make that jump, or Lara Croft always knows the specific language needed to open the door puzzle; because it'd be a pretty boring game otherwise. "Oh, there was this really cool thing that happened last year. Don't worry about it though, Elminster dealt with it." Fun.