Why would any creature with reasonable intelligence or even a modicum of common sense choose to live in such a terrible environment? Some races have lived here for generation upon generation, and the Lowerdark is simply their home. Other creatures settle here to take advantage of the Lowerdark's unique magical properties, rare ores, or shelter from the hated sun. Still others view a sojourn in the Lowerdark as a temporary solution, since the dead magic areas and hostile territory may be a wanted criminal's most expedient means of avoiding capture.
Shield dwarves have been known to establish temporary mining camps in the Lowerdark to wrest particularly valuable ores from the bowels of the earth. Those who thirst for knowledge or lust after archeological treasures often find the Lowerdark tempting as well, since the remains of the first empires of Netheril and Imaskar lie here. Still others have personal or philosophical reasons for staying in this inhospitable area.
Of course, not everyone is in the Lowerdark by choice. Some unfortunates are here because they neglected to research the destinations of the portals through which they ventured. Others have been exiled here from communities in the Middledark, the Upperdark, or even the surface world. Some drow matrons get rid of potentially problematic subordinates by sending them on exploratory or expansionistic raiding parties into the Lowerdark. A triumphant scout returning from a mission into the depths might find her unexpected survival fatally inconvenient to the matron who dispatched her.
Several Middledark races make frequent forays into the Lowerdark. Some do so because of expansionistic desires, others because they need the resources, and a scant few because they have something to prove. Grimlocks and orogs often set a coming-of-age trial for their young warriors to mark the passage between adolescence and adulthood. In a typical version of such a trial, the youth is sent into the Lowerdark, sometimes armed only with a dagger, and told to return with a trophy demonstrating his competence and ability to contribute to the community.
Various intelligent creatures have formed small settlements in the Lower Underdark. All these communities are fine places for adventurers to explore.