OpenSUSE itself could be seen as a fork of Fedora though it is from long enough ago that perhaps that is not fair.

In the beginning there was Slackware ( well, maybe SLS but it is gone now ). Slackware has no packaging system. Most distros want one. Debian is not really a Slackware fork but it was a response to it.

The first two distributions to bring true package management were Debian and Fedora ( well pre enterprise Red Hat really - before Fedora ). So Fedora and Debian are the classic bases for other distros.

Red Hat created the first, and most successful, “enterprise” distribution so lots of people want to clone that.

Ubuntu was the first distro to really succeed at a “mainstream” desktop experience. Ubuntu is itself a fork of Debian but, because of its early success, there are probably more forks of Ubuntu than anything else.

Arch was the next really successful attempt at a new packaging system and a distro for more technical users ( that still wanted a binary package distribution system ). There are forks of Arch but, as the repos are Arch’s biggest strength, few of them deviate too much beyond the installer and default configuration.

If you are going to create a distro based off another one, it is typical to start with the base distro that is how to the package format of your choice.

There are now other distros with their own packaging systems ( eg. Alpine and Void ) but they have not been around as long.

Importantly I think, OpenSUSE is European base and, for a lot of the history of Linux, it was largely an American phenomenon ( yes, I know it was invented in Finland — how long did Linus stay there? ).

Finally, “forking” is often a little more sophisticated now. In the past, you started with some other distro that you liked and changed a few small things that you didn’t. A lot of that is taken care of with repos and spins these days so you so fewer unique distros start life this way. The exception is maybe init systems.

As an example, you could consider Chimera Linux as a successor to Void but it is not really a fork. It uses a different package manager, userland, and init system for example.

OpenSUSE itself has multiple versions. One of them is likely to be close enough for fans that they do not need to splinter off.