I'd say two things, the first is that if you're not paid, you don't have that much incentive to contribute hardly so it's no surprise that most OS projects are maintained by a handful of (great indeed) highly motivated developers. Second is that it's something widely known that developers are not flexible, so if they're in charge of their baby, they won't make so many efforts to encourage outsiders contributions. Whereas in a corp environment, managers and other devs should make sure it doesn't become an obstacle to contributions.

That's not very precise but I think people can relate to what I said.