Sorry, I didn't really understand the parents as that. I guess a more on topic reason might be:
They can barely manage a team of their own engineers to get this product out. Throwing more guys at it isn't going to help necessarily. Not without significantly more overhead and more managers. I think mystical man month definitely still applies to open source projects as well.
that and microsoft is just not good at managing open source. And I can't imagine anyone would want to work on the windows kernel without being paid to do so >_>
