I am referring element number 702 4x4 corner brick, and possibly other elements that have not been in sets in a considerable amount of time.
I've been wondering the reasons why a number of pieces are no longer available and it appears the decision to reduce the overall amount of different pieces is simply a cost based one.
From a Q/A with someone from the Lego Company: "...the piece count has been reduced drastically and there's a move back to roots in Lego, not only for creativity but to save money. Lego went from 12,000 different pieces to 6,800 in the last few years-a number that includes the color variations."
snippet was from here: http://lego.gizmodo.com/5019797/everything-you-always-wanted-to-know-about-lego