As mentioned else where, They ran out of time. ToB was supposed to be a full 3rd game rather then an x-pack, but WotC was pushing them to make it a 3rd edition game instead, which would screw a bunch of stuff up, and they opted to try and end the series as cleanly as possible before the deadline for the 2nd ed stuff ran out.
There's still a lot of room for improvement though. The reason ToB is pretty horrible is because of how linear it is and it's very rushed pacing. Adding some new areas and side quests, and allowing for exploring Tethyr's history and culture would greatly expand on what's already there (It would also have some potential to explore Jaheria's history since her family were noble supporters of the royal family of Tethyr, prior to the coup that left her orphaned and in the care of the druids, but I think that's one of the things that they're required to be hands off about). At the very least they could do mostly self-contained areas like SoA did. With or two quests extending to another area. Or add the possibility after Oasis to go say "Hi" to the current rulers of Tethyr, to "thank" them for their most "gracious" "welcome party".
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There's still a lot of room for improvement though. The reason ToB is pretty horrible is because of how linear it is and it's very rushed pacing. Adding some new areas and side quests, and allowing for exploring Tethyr's history and culture would greatly expand on what's already there (It would also have some potential to explore Jaheria's history since her family were noble supporters of the royal family of Tethyr, prior to the coup that left her orphaned and in the care of the druids, but I think that's one of the things that they're required to be hands off about). At the very least they could do mostly self-contained areas like SoA did. With or two quests extending to another area. Or add the possibility after Oasis to go say "Hi" to the current rulers of Tethyr, to "thank" them for their most "gracious" "welcome party".