Rebel Code (http://books.google.com/books/about/Rebel_Code.html?id=kIU1s...) does a pretty good job summing up how Linux migrated from tarball source releases to BitKeeper (and eventually, of course, git). It appears that Steve Weber also writes on this, in a format that may be easier to read. (http://books.google.com/books?id=78SLSiWqy14C&pg=PA117&lpg=P...). The e-mails sent at the time around this make this thread seem, well, pretty friendly, actually. (http://lkml.iu.edu/hypermail/linux/kernel/9809.3/0493.html)
To the Postfix maintainers' point, Linus appears to have had quite reasonable reasons for maintaining the changes himself in the years up to then. Only when the number of active contributors got large (I suspect significantly larger than Postfix's) did this model break down. It's not the best for everyone, but I'm really glad they're maintaining a great open source product. The early 2000s in particular were not an easy time for MTAs.
edit: fixed 2 pretty big typos.
The need/usefulness of version control was orthogonal to (and preceded) the decision to write his own (d)vcs.
(Edit: misrememebred the actual vcs, originally posted Mercurial instead od Bitkeeper - http://en.wikipedia.org/wiki/BitKeeper#History )
Here's the link to "A short History of Git": http://git-scm.com/book/en/Getting-Started-A-Short-History-o...