And they couldn't even get Kevin Michael Richardson on board to reprise his role as Sarevok.
Though, given how butchered he ended up being in the game, that might be for the better. He's just there as a bossfight, and that's the extent of his role. Are you saying that nobody in a city with a powerful military force / adventurer presence has ventured down into the undersewers in 100+ years and cleaned the Bhaalist mess up, especially after Sarevok has already done a ton of damage once?
Same with Viconia and the Sharrans. You'd think there'd be paladins on the hunt for a sect of a generally shunned goddess in a city which works to maintain law and order (unlike somewhere like Westgate which is an actual cultural and racial and religious stew), but I guess that would imply that there's clearly defined "good" and "evil" in Toril, and we can't have that nowadays, apparently.