This was in no small part due to the large number of slaves the house sold in Skullport. Using a portal to their outpost of Kyorlamshin, led by Matron Tanor'Thal's niece of Kesra, they captured any potential slaves that they could and sold them in the bazaars of Skullport. House Tanor'Thal was, in fact, Skullport's most prominent drow slaving organization.
Political pressure was being put on Matron Tanor'Thal back in Karsoluthiyl to allow access to Kyorlamshin's portal for the other houses of the city. This was a concession the matron mother did not wish to make.