I don’t think there’s any one thing doing it. It’s a staple of the genre. It’s an ingrained part of the pop culture concept of elfy-ness.
I have never been swayed against a race/class combo. No matter the combo, even with point buy you can start with a +2 in your main stat which is all I really feel you need.
What's not optimal about a half orc paladin? Charisma is not super crucial to a fair number of builds. You are not generally casting spells with saves. They have strong racial abilities that help with damage or defense.
I would not really use one to make an Oath of Conquest Paladin, but they pretty strong.
Soon as you hit level 4 you can boost to that +3. I find there isn't that much difference between them in play due to the swingy nature of the d20.I can agree that +2 is playable. +3 is highly effective though. It's a big enough difference that it gives me pause in creating a +2 main stat character.
Why is there some sort of 'value' placed on playing against type, or playing an 'uncommon' combination? Why is there some sort of judgement value placed on 'optimizers' who just like having a +3 to their damn main stat exactly?
Why would being able to play a 16 STR Halfling Barbarian make it any less surprising IN FICTION that the character is a dangerous barbarian?? It's not like we're playing a MMO and all the tables are in the same universe or connected somehow. If your PC use alternate racial traits, then that could just mean they are special IN UNIVERSE as well. What's the story behind that Halfling? Why is he so fierce? Was he raised by a clan of Goliath? Or maybe just in a rough and tumble neigbhorhood and decided not to let the Twiceling walk all over him? PC options don't have to become the norm.
Pretty sure Warlocks are way rarer in universe than they are among D&D players, for exemple.
But having to be high-charisma to be a competent warlock or sorcerer has always bugged me. Surely an archetypical warlock character concept is the surly resentful unpleasant type who bargained for power to get back at everyone who shunned him? And sorcerous power is generally meant to be hereditary - I can't see any logical link there that means all competent sorcerers should have great people skills. And this one CAN make a concept unviable. If I want to play a miserable smelly self-pitying warlock who is tiresome to be around, I either have to come up with some tenuous rationalisation as to why this guy is particularly charismatic, or else grit my teeth, shove that 8 in Charisma and resign myself to the fact that his social ineptitude means he's doomed to be just plain bad at warlocking for his entire career.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.