Environment. Culture. That is, the Character's History.
While an elf could sell slaves the same way as a drow might, they would be less inclined to do so, due to upbringing and therefore, the drow slaver is a more plausible character concept.
Human demon cultists are not very common, but that's not the only thing. A Tiefling, especially one who spent most of his life on Toril, might have an entirely different outlook on life than a human, due to his appearance, and society's reaction, and therefore would be more inclined to be evil.
Imagine putting two people in the same exact environment, the same culture, the same people, but having one be a human and one be a tiefling. The human will grow up normal and be accepted into the community (most likey). The tiefling will be a social pariah, always scorned and hated for his scaly skin and brimstone stench. Because of that, the tiefling is more inclined to hate other people. The idea behind subraces, is for making certain concepts more plausible than others.
Anyway, the same examples go for the dwarf vs duergar, and aasimar examples.
Dwarves face huge pressure from their society to grow up honorably, while a Duergar society is far more inclined to chaos and a cutthroat attitude.
An Aasimar is essentially 'closer to god', because of his birth, and so, as the others, is more inclined to smite evil. A regular paladin is perfectly normal, but the boy who goes to the church to become a paladin may have been -less- inclined to do it.