I would guess that it has something to do with the increasing value of the YCombinator brand name. In 2006, a company that is already earning good revenues isn'

I would guess that it has something to do with the increasing value of the YCombinator brand name. In 2006, a company that is already earning good revenues isn't going to give PG a chunk of equity in exchange for $15,000, some free meals, and advice. In 2010 a YC investment still offers $15,000, free meals, and advice, but it also guarantees that you will catch the eye of top flight VCs, be part of an alumni network laden with talent, and get great publicity. That's appealing even to companies that have an established product.