IMO it's also the release model, which follows the even (stable) and odd (unstable) version numbers. When 2.10 was released there had been lots of unstable vers

IMO it's also the release model, which follows the even (stable) and odd (unstable) version numbers. When 2.10 was released there had been lots of unstable versions prior to that, but as they're marked unstable (and can be) distributions will never pick them up. Like this, a really long time gap can pass between two stable versions even if the code has been under active development (remember the Linux 2.4 -> 2.6 transition?). Of course it's a lot of speculation on my part, and no, I'm not advocating for time-based releases (they IMO make no sense for this type of project).