While I would like to believe it's because YC-funded startups are better, the fact is that for any startup to raise Series A, the VCs have to believe they're so good they could one day go public. So my guess is that the reason so many YC alumni have been able to retain control is that they are so well connected. They have the other alumni (many of whom are very sophisticated about fundraising) to give them advice, they get hooked up with the best lawyers, etc.
Now that is one area the academic literature on venture capital and entrepreneurship hasn't studied: the social networks of entrepreneurs and bargaining power. Putting that on my list.
All these things that YC-funded startups have going for them (screened teams and ideas, network, support, etc) probably create more demand for them from VCs, giving them better leverage than non-YC-funded ones.
