Is there an in-game explanation as to why some races can multi-class and others can dual-class? For example, is there an explanation in an article or book somewhere saying that It's the will of the gods or whatever that a half-elf can be a druid/ranger and an elf can't?
I am aware (of and unconcerned with) the game balance concerns raised by the Player's Handbook. I know about those. I'm looking for narrative reason rather than a mechanical one.
Best Answer
The Player's Handbook (1989) for Advanced Dungeons and Dragons, 2nd Edition says
The Dungeon Master's Guide (1993) for Advanced Dungeons and Dragons, 2nd Edition says
Conclusion
So the ability to multi-class is, in essence, paid for mechanically through demihuman class and level limits and justified narratively by saying that humans possess greater ambition and drive than demihumans. This can be lumped together: human drive limits humans to dual-classing rather than multi-classing because humans throw their all into things.
(Going into this, I'd thought at the time that the humans' short lifespans were part of the the reason that they couldn't multi-class, but that detail's absent from the Dungeon Master's Guide and goes unmentioned in the entries for shorter-lived races in the Complete Book of Humanoids.)