If I remember correctly in AD&D humans did not have any advantage while the non-humans had all sort of bonuses unlike in 3.5, so Gygax gave them level caps in order to keep the world human centric. Why would you play a boring human if elves had sleep resistance, dark vision and could spot hidden doors without searching for them.
If you did not plan for a long campaign then playing as an non-human was not only ok but desirable.
If I remember correctly in AD&D humans did not have any advantage while the non-humans had all sort of bonuses unlike in 3.5, so Gygax gave them level caps in order to keep the world human centric. Why would you play a boring human if elves had sleep resistance, dark vision and could spot hidden doors without searching for them. If you did not plan for a long campaign then playing as an non-human was not only ok but desirable.