I vaguely know that the backstory of the Drow from earlier editions involves their Sunlight Sensitivity coming in part from the curse that banished them, and not purely from their long existence underground (though if I'm incorrect on that, let me know- I seem to recall WOTC staff mentioning things along that line recently, but I believe 5e sources haven't gone into that specifically).
Is there similarly a lore-given reason for the duergar's sunlight sensitivity, or is it simply attributed to their history of dwelling deep underground? Them having Sunlight Sensitivity but the svirfneblin not having it made me curious.