The Monster Manual entry for Elves mentions it, and states that Drow do not get the usual elven weapon proficiencies. This is because Drow usually dwell in the Underdark, where Longswords and Longbows aren't as useful in cramped spaces.
I can't find the Drow replacement for melee right now, but I know Drow usually use the Handcrossbow instead of the Longbow, so they might have an automatic proficiency in it.