I think the 'easy' answer is liability, same as it is for any other complex human engineering achievement. Liability though would mean at the very least allowing commit access to only identified individuals and companies that are willing to pay for insurance coverage (to gain commit access).

This would probably ruffle too many feathers from the GNU old-timers, but I really dont see any other option. We are way past the tinkering-in-the-basement days of Linux/BSD hackers when most of us just wanted a cheap Unix box to play around with or to avoid Windows. A massive percentage of the civilian (and other) infrastructure is built on the shoulders of unpaid hobbyists. There is already massive liability at the social and corporate level. Time to deal with it.

EDIT: Ok, sounds like I have to describe this better: 1) you (governments) force commercial providers to assume liability for security issues and exploits and force disclosure, etc, 2) their insurance premiums go up, 3) to reduce premiums they only use checked/secured software, 4) that means maintainers of at least the critical pieces of software get paid via the (new) channel of risk reduction. Doesnt apply to all OSS, doesnt even apply to all distros. But it creates an audit trail and potentially actual compensation for maintainers.