A Typical Member of the Species
The main reason is aesthetic. Humans as the core of the setting just feels right. If most of the characters in the setting are strange races, those races aren't strange anymore. Sometimes I question if there's a racist subtext to this attitude, but that way lies madness. All of D&D has an awful racist subtext if taken too seriously. This race is "usually chaotic evil." Are they born that way? If so, shouldn't we kill them all? On the other hand, does their society make them that way? If so, how can we reform that society? etc. That's pretty far off the rails. All I'm saying is I feel like most player characters should be human to ground the setting a bit in a single cultural perspective.
I know that last sentence will grate on some D&D players. "Variety is the spice of life!" Many people will play a race just because they've never played one of those people before (a bottomless cauldron of races exists in later editions of D&D). There seems to be a belief that more races means more story and role playing potential.
My answer to that? Almost all of the incredible stories of the literary canon include one race (human) and one class (fighter, with almost all of the others being 0-level or commoner types). Race and class alone do not make for good story.
Then Again, This Is a Character From Shakespeare...
In fact, I think some of the variety of racial choice becomes a crutch for role playing. "I'm an elf, so I hate dwarves, and I'm aloof." "Well, I'm a half-orc, so I'm big and dumb." Not everyone who plays non-humans does this, obviously, but it does crop up too often for me. I think human characters require the player to step at least a little further away from cliches and put some thought into who their character is.
Favorite human characters:
Justinian, the 1e AD&D cavalier paladin who found a pool transmuting metal to platinum in a randomly generated dungeon. He and Vedron became fabulously wealthy and retired thereafter
Shelby, the 1e AD&D illusionist/magic-user (probably not kosher, but man did he have a lot of spells) who left the comfort of his plush family estate to fight frost giants in G2.
Jean-Pierre, the 1e AD&D PHB paladin who fought the hill giants in G1, fought the frost giants in G2 before being captured by ogre magi and sold into slavery. He was later rescued in D2, before being promptly gutted by a nameless kuo-toa
What's your favorite playable race? If you're doing the 30 Days of D&D Challenge, link your blog in the comments so I can follow you.