What is it about Debian that caused the proliferation of derivatives?
1) They nailed package management early on. Apt doesn't feel revolutionary today, but in the 90s it was WAY better than what Redhat was using as it's package management core.
2) Debian's staunch adherence to their free software guidelines drives forks in two ways. First, you can fork without any worry you're violating a license on some component. Secondly, it encourages people to fork because some people will want to include some of those missing components in out of the box installs, and that simply won't happen with Debian.
3) Ubuntu. If you look at derivatives, about half of them forked off of Ubuntu, so Debian gets to count them as descendants because Ubuntu is based off Debian.
And APT (with policy) still blows RPM out of the water. Lack of policy is a big part. I'd like to play with Mandriva, which uses APT tools but the RPM format just for comparison, but the fact that I can pull apart archives using ar, tar, and gzip, as opposed to relying on an inconstant binary format, has proven invaluable more than once.
Ubuntu is not only based on Debian, it still regularly merges in packages from Debian.
They should make some kind of distinction between a release that is based off of the parent, but then goes its own way, vs a release that IS the parent + additions. The relationship is much closer.
And actually it goes the other way too - debian tries to merge changes from ubunto back into it. (They have a tool that automatically compares them and suggests merges.)
Still very much the case. Their ass-backwards networking setup scripts, as just one example.
slapt-get
Also, Debian was not dependent on a corporate master, so by basing off of Debian you could be confident that the rug could not be pulled under you by a competitor.
The 3rd reason is that a lot of recent distributions are based off of Ubuntu, so they necessarily become Debian based distributions as well. Basing off of Ubuntu gives you the best of both worlds: you get the cadence and corporate backing, but less of the risk -- If Ubuntu starts playing hardball, you can relatively easily transition to a Debian base.
- simplified the installation process
- narrowed the focus - switching away from the perception of Debian being a 'jack of all trades, master of none'. To give an example, the initial release of Ubuntu only supported a couple of thousands of packages on 3 platforms, versus Debian's tens of thousands of packages on 8 platforms.
- increased the release cadence
However, it ended up shipping late and RedHat derivatives got a foothold. If it wasn't for Ubuntu, it would have never fulfilled it's original intention at all.
Also, since I'm commenting here, that chart shows a whole lot of completely dead distributions continuing onto the right. It would be a lot more interesting to see how long these forks actually lasted.