Strange indeed, I've noticed several medium sites that are not included as well. It seems that Y doesn't want any site in their index that could directly compete.
Why send traffic to competitor, as if that's the mentality of this SE but they themselves have over 17 million URL in Google. That's a lot of potential free traffic send by G to Y.
I'm kind of incline to believe that this is a commercial database...paid for inclusion...and those who can't afford could not be listed, likewise to competing ecommerce site.
Basically, this is not the SE we are hoping for that could challenge Google but some kind of glorified Inktomi deal.
I hope I'm wrong