I think the economics of oversales have caused this.

My theory is that in the past, for the airline, overselling the plane meant gaining a $2,000 ticket and having to pay out $200 - $400 in voucher, it was a good deal for the airline, making it almost foolish not to oversell for each and every extra seat they could. Now with last minute fares much, much lower in so many markets, the economics of the oversale change, making it less worthwhile.

Plus, with load factors constantly going up, the carriers have ever increasing experience with full flights, and they're getting better at "right-sizing" the number of seats to sell.

A few years ago, running into an flight oversold by 10+ was frequent, whereas now, when they are oversold, it seems to be by single digits (sometimes low ones at that).