A mage could be drawn into a drow city by scholarly fascination, or by the rumors of strange new spells or enchantments that reside deep below.  There are a multitude of reasons why a wizard's curiosity would take him to such a place.  Most mages are, by their very nature, eager for power...how else to explain the years it takes to master the necessary hand movements, archaic languages, and concentration to master their chosen Art?