I think it was explained that after thier barbarian guide got chopped up in the Statue trap with the sliding porticullies that Nualia and her cohorts began to tread more carefully in that section of Thistletop.
Since her current resources were A) a quasit trapped by her own design in the Ctacombs,
B) A halfhearted merc,
C) a foolish research petty wizardess,
D) a horny bugbear,
E) and a millenium mad Barghest sending dreams
it might not be beyond reasoning that thier physical exploration would stop there, until the magical hindrance was dealt with.
