Making playing a human appealing to play

In pre-2000 D&D and its close relatives playing a non human (demi-human or humanoid) conferred specific ability or stat bonuses that exceeded anything a human could achieve. On the other hand humans were the only race to have access to all classes and no limits on how far they can advance in those classes. So a dwarven wizard or halfling paladin is out, and a human thief will always be capable of higher levels than an elf, even with the elfs longer lifespan.

This is not a horrible solution, but it can restrict the potential roleplaying opportunities though admittedly there are almost as many or more ways to munchkin out a character. A dwarf with arcane powers is a fascinating concept for example but as they are written up in 1e/2e AD&D they have some rather brutal resistance to magic as they progress in levels.

This leads to question of direction, do you give humans a bonus or restrict demihumans? One of the key concepts of pre 2000 D&D is the fact that the world is human dominated. If demihumans can be anything what is to stop them from overshadowing the humans? An incredibly durable dwarven paladin or the unearthly dextrous elf thief will achieve things over their longer lives that a mere human could, on a purely mechanical level. A wonderful example of this and the various mechanical tweaks tried to fix it is the halfling. As written by Tolkien a halfling does not belong in an adventuring party, because they simply arent suited for it. And culturally, going out adventuring would be treated like a sign of madness. So as the editions increase, they begin to gain additional abilities like luck bonuses until we have halflings that are more like Belkar from Order of the Stick. And the Baggins were never "shoeless gods of battle".

If we give humans bonuses to offset the fact that their mechanical stats are inherently going to be lower, you either give them equal stat bonuses and thereby cheapening the stat bumps that the other races have or an alternative bonus. Perhaps the reason the humans are the most populous and powerful race (besides breeding like rabbits) is the short term view that humans have that motivate them to push themselves hard.

If instead we restrict demihumans then we are limiting roleplaying opportunities, but the ones left are simpler to handle mechanically. The different races then become focused on the racial niches they've carved out. Such as an elf being a mage rather than a barbarian or a dwarf who fights rather than slinging arcane magic.