The actual answer is a metagaming one, and one you have to reach back into Gary Gygax's first edition AD&D Dungeon Master's Guide to get. He made Humans dominant because they are the race we most easily relate to. The first wargames that became D&D games from Arneson and himself assumed humans were dominant because we're the ones who have the wars (not to sound patronizing). He carried that theme because of the theory that fantasy like sci-fi can be best experienced through a human frame of reference for human beings. Therefore, humans are the ones who can advance unlimited levels in AD&D, who don't have ability score limitations, etc. (Ignoring the female STR limitation thing for a second - that's another issue). As fantasy lit has progressed, we fans are quicker to take to something that does not express something from a non-human frame of reference, and we happily ignore previous limitations on ability scores, etc.
Now, if you're talking about why so many people make D&D human-centric today, I'd say that trend is changing. Look around these forums; look at Eberron, look at the FR revisions of 3E, and I think you'll find the trend is reversing; elves are no longer in retreat, certain races are co-equal, heck in Eberron most of the Dragonmarked houses aren't even run by humans! It's a trend with deep roots that is recently coming to an end, or at least an end of the monopoly.