Also incredibly important on top of that, lots of people had a personal interest it succeeding:
- Red Hat, Novell etc on the software front.
- Also companies like HP needed a *nix and BSD was currently engulfed in FUD.
If you want your product to succeed you're far more likely to be successful when other people can profit as well from your project's success. "What's in it for me?" is the rally cry of any potential investor of time or money. In the case of Linux there was a lot in it for a lot of people.
The people who most profited from BSD (AT&T, IBM etc) were dinosaurs who history has proven have been bad at reacting quickly, at least in the 90s and early 00's. IBM is getting more nimble but the parties who profited from a successful BSD turned out to be the least suitable advocates.