Humans were the baseline of the game - and even then they had a pretty WIDE spread of differentiation. Demi-humans and other playable races had to fit into the spectrum of what was pragmatically considered within the human range. (e.g. bigger than a quark, smaller than Jupiter), but they could and were considered to extend outside the starting playable range. Those who did remained NPCs.
The conceit was that humans, you and I, were the ones who were going to play the game, so this baseline gave us a starting point on what to expect. Sort of like how all the Players worked together best if they shared a common language (not just their characters).
Over the decades humans stopped being the baseline and became a sort of Generic. Humans are +1 this, +2 that, while everyone else is vastly more defined. They have flavor, we get dull, dull, dull.
I *believe* D&D next is taking an "always positive" approach to game design. Instead of having the baseline be human and demi-humans vary with both drawbacks and advantages, we get a non-referential baseline and every race defined by their list of additional benefits. That feels good, but humans are still generic and poorly defined. Part of this is because of the baseline, but I think another part is the benefits-only approach. We don't want to define humans, at least not too much, but defining us with another creatures as the baseline only makes it more difficult to relate to the material. Plus, I like having drawbacks. Half the scores on the 3-18 bell curve usually fall below the average. That should be more than playable, but in any case it is interesting.