I would assume that the Charisma bonus is because they come from a cut throat political society where you need to know how to talk and navigate society in order to not get dead. That's the explanation I can think of anyway. Or maybe there's some kind of inborn magic at play.
I think Charisma is a bit of a misnomer in 5e. It’s less about how charismatic you are and more about how much you are able to influence other people (i.e persuasion, intimidation). Tieflings and drow get Charisma bonuses because they are naturally intimidating and so can get people to do what they want. It’s also the reason why a lot of the mind control spells require Charisma saving throws. It requires you to resist the caster’s influence. In fact, I think Influence would be a better name for Charisma.
