I think you are all missing the point. Since a 10/11 is by definition, AVERAGE, the answer is that it is the numbering that has changed, not the scores.
On a less meta note, however, it is naive to think that Intelligence, Wisdom or Charisma are somehow less prevalent now than they were at any point in the past. If people are dumb now, it means everyone is the past was just as dumb (on the whole, of course). Strength and Constitution are the only ones that are debatable. Of course, D&D does not model fitness well, if at all. But I suppose you could say that both Str and Constitution are on the decline, for some of the above reasons. Do not forget, however, that Constitution also takes into account resistance to poison and disease, which is arguable up in some respects and down in others. The fact that we are being continuously poisoned to some degree by our very environment means that our Constitution is probably hurting as a result of being damaged over time...