The Bell Labs approach to source control was, I'm, weak. It relied on snapshots of the tree and out-of-band communication. Don't forget how small and tight-knit that development team was, and how valuable perfect historic snapshots were.
Add that 40 years ago source code revision control systems were incredibly primitive. The idea of an atomic change set (in Unix land at least) was revolutionary in the early 90s.
This is one place where 35 years of evolution in software practices has very much improved.
Paul