The way I've always had it, Llolth killed off any other deity the Drow ever tried to raise against her, thus ensuring she's the only in-culture one they've got. Ditto with Gruumsh and Orcs.
A Drow could, of course, go out-of-culture to find a deity, but that would pretty much amount to self-exile.
Oh, I most certainly am not.
Same as Dwarves are PC-playable but Duergar are not, or Gnomes are PC-playable while Svirfneblin are not, Elves are PC-playable while Drow are not. Each of the non-PC species are the "underdark mirror" of their surface equivalents.
That said, if the game was set full-time in the underdark I'd be tempted to reverse all that.
Okay, but do you not see how all of this still comes back to "this is the Lanefan-specific thing"?
Someone coming to this with no knowledge except what 5e is (whichever version, both include playable drow) wouldn't know these things. Meaning, as I already argued, explaining yourself is kinda important. You need to
get the player on board, not shout at them that you're pulling rank and they'd better fall in line.
Side question - and this came up at our last session - what's the underdark mirror for Hobbits?
AIUI, formally, there isn't one. But for my own personal concept, I would say Ghostwise halflings.
I get that for Drow in particular, which is part of the reason they're pretty much gone from my current setting. But other monsters IMO should be just that: monsters, until and unless an individual proves itself otherwise.
Okay. The problem is, several of the species that
have been "monsters" really do look exactly like "demonized groups of IRL humans". Orcs are another good example, because (unfortunately) from their very inception, they've had some unfortunate Orientalist tropes woven into their story, and a big part of why they're getting a huge injection of nuance is...we'd like to keep what makes orcs cool, without making them "see,
these humans are okay to murder indiscriminately because we asserted that they're a different species, and they're green!" Because that has unfortunately been an all-too-common failing of human nature across history.
Orcs, for example, are usually kill-on-sight in my game, but tell that to the party who ended up taking surrender from some Orcs then taking those surrenderees in as paid party henches. Those Orcs turned out to be loyal to a fault - all they needed was someone to treat 'em right - to the point where the party Ranger (who mechanically gets combat bonuses against Orcs) ended up leaving the party to join the Orcs and become their liaison with the neighbouring Humans.
Which is super cool! I love that stuff.
To add perhaps a bit of clarity to my previous stuff, "monsters" include things like mindflayers (inherently horrible creatures that literally need to murder to live), undead, constructs, vampires, and things like mutants. Now, one
can change the concept and demonstrate the truly inhuman nature of something that hasn't been super great in the past, e.g. actually
showing that one's "orcs" are grown in labs and programmed by fantasy computers or the like. (Though even then, there's something to be said for "why does it need to be 'orcs' then? Couldn't it just be human clones altered to be mindless shock troops?" Aka, one can do a lot with pig leather, but there comes a point where one must recognize it won't make a silk purse.)