I suspect the licensing played a role in it, actually. With Linux being GPL, no single vendor could - in theory at least - take the code and make a proprietary fork. It became a neutral ground, so to speak, where corporate contributors could cooperate.

Plus the lawsuit from AT&T came at a critical time, and of course pure chance played into it as well - Linus has stated that he was unaware of the BSD projects when he started Linux, and that if he had known, he probably would not have bothered with writing his own kernel.