Though I can't be sure, I think a big reason for his retirement had to do with the Two Minutes Hate he was subjected to at the hands of the Internet Outrage Machine. PG has always had a knack for being misunderstood, and I think disappearing from the spotlight was his way of making sure that problem didn't hurt YC.
Unfortunately for you and me, that means no more great essays. It's kind of funny because, in retrospect, "What You Can't Say" almost prophesied how this all went down; his only mistake was not seeing the storm coming.