The Tangled Web She Weaves
The Time of Troubles was not unanticipated.
In fact, many of the gods, including Mystra, Bhaal, Selûne, and Shar, were well aware that something serious and potentially dangerous would happen soon. For many of the deities of Faerûn, "soon" could mean in the next second or the next millenia, but the dark powers allied with Bane and Myrkul in their grab for power, knew that "soon" meant within that same century--what the humans of Faerûn referred to as the 14th century DR. Scarcely did the evil deities of Toril and its Planes converse, but word was passed from Bane to Loviatar to Malar and eventually to Lolth.
The wicked Spider Queen, chief goddess of the evil drow elves of the Underdark of Toril, had been conspiring for thousands of years to gain even a fraction of the power accorded to the deities of the surface races, and her plans to secure the power of spellfire for her own worshipers had had a critical success. By cunning and craft, Lolth stole the gift of spellfire from a wild elf child it was destined for and spirited it away to the Abyss. Upon learning that the exile of gods would occur "soon," Lolth decided to secure the volatile power from her enemies the only way she believed possible: give it to a drow child.
Faerzress, the mysterious magic of the Underdark, functioned in strange ways, and Lolth was not certain if spellfire would work properly in the lightless world her worshippers called home, so she immediately ruled out giving the power to a promising female child. (After all, she always needed all the clerics and priestesses she could get.) But would giving the spellfire to a commoner be feasible?
Finally, Lolth decided that the power of the spellfire would go to a male child of noble birth. As a general rule, Lolth despised males, but found the occassional one useful for means beyond siring children. The many wizards of Sshamath were among the most powerful males of Lolth's realm, as were Gromph Baenre and Jalynfein of Menzoberranzan. Despite the undisputed magical talents of the males of the larger city of Sshamath, Lolth fervently decided against choosing a child of a city not ruled by a proper matriarchy. Ched Nasad hadn't any recently-born noble males of sufficient rank and intelligence for the task, so her demonic gaze fell to Menzoberranzan.