I would be willing to bet that more and more folks have become educated on the secrets of getting bumped (i.e. getting on the volunteer list as soon as the gate opens) so that nearly ever flight has at least one person willing to volunteer. If their oversold by one, there is no need for a formal announcement over the PA asking for volunteers because they oversold their flight(which marketing folks realized wasnt the best PR in the world anyway)... That and the fact that airlines have gotten better at predicting the overall no-show rate.