Git

This glosses over what is, to me, the most fascinating part of the history.

SVN was, along with its proprietary contemporaries like Perforce, a pain in the ass as far as branching and merging and collaborative workflow across more that one branch of development or one machine installation. It worked, kind of, but for something at the scale of the Linux kernel it was simply unmanageable. The distributed nature and scale and speed of Linux kernel development, and the cantankerousness of most of the kernel team, meant that they weren't willing or able to put up with any available solution while their project was progressively scaling up. And therefore, the version control system used by the team for one of the most advanced pieces of technology in the world for quite a lot of its development, was... diff and patch. And email. Linus just kept separate directories for separate versions, with the names of the directories corresponding to version numbers, and every single change that needed to go into the kernel would get emailed to Linus (or emailed around developers who were collaborating on "a branch"), and people applied diffs manually in order to pull changes.

That sounds like a joke, but it actually made more sense than anything else they could do. The workflow of a pull request, with no necessity to maintain a shared source repository which was centrally updated, simply didn't exist in any version control system at the time. diff and patch and email could do that, and it was quick and easy for small changes and possible (with some discipline) even for large changes. Everyone sort of knew that it wasn't ideal, but it was literally better than anything else available.

So, bitkeeper. It was a little weird for such an open-source darling to switch to a proprietary VCS, but its creator and CEO Larry McVoy was a respected kernel developer himself, and he was basically offering something for free that would be a significant step forward. Bitkeeper supported a lot of modes of distributed operation that were unique among VCSs at the time, informed by McVoy's own firsthand experience with working on the kernel alongside everyone else, and so it was generally regarded as a step forward technologically. From around 2002 to 2005, the kernel used bitkeeper as its primary source control.

It was a little bit of an uneasy peace, though. Some kernel developers were unhappy with having the source and all revisions "held hostage" within a proprietary VCS, and wanted to have the ability not to use Bitkeeper. McVoy, for his part, didn't do himself any favors by being kind of a pain in the ass at times. A central point of contention was his objection to people "reverse engineering" elements of the BitKeeper protocol. Some developers wanted to be able to interoperate with bk without having to license McVoy's software. McVoy said they weren't allowed to, and said they could have a license to his software for free so what was the big deal. Things came to a head when Andrew Tridgell, a developer who did have access to bk, was accused of "reverse engineering" and had his license (and hence his ability to work) removed. Everyone got mad, arguing about Bitkeeper substantially eclipsed useful work on the kernel, and the need for a better solution could no longer be ignored. (It was later revealed that Tridgell's "reverse engineering" was to access the Bitkeeper endpoint with telnet and type "help".)

Linus, in his inimitable fashion, decided to solve the problem by putting his head down to create the solution and then dictatorially deciding that his solution was going to be the way going forward. It may not be in people's memory now as clearly as it was back then, but he was (and is) a genius wizard-man at systems design and architecture, and obviously his word was basically the word of God on significant matters within the kernel community. Basically, he took a lot of the concepts that made BitKeeper work, and put them within a lot more clean and general architecture, spent a few weeks making a working prototype with a lot of feedback and testing by the kernel team (which by this point absolutely badly needed the whole thing to work just so they could do their day-to-day development again without any fistfights), and then said, we're switching to this, everyone install it and shut up and stop fighting please. Over the succeeding years it was so clearly leaps and bounds ahead of any other VCS that more or less everyone in the world switched to it.

Moral of the story? Let people be. If McVoy had been a little more accommodating, maybe we'd have BitKeeperHub today, and he'd have been able to collect license fees from half the developers on the planet because everyone wanted to use his still-superior-to-everything-that-exists solution.

(edit: Added the details about Andrew Tridgell's reverse engineering)