There seems to me to be a pattern for how many Linux distributions gain prominence and then fade away. Here is a family tree up until 2012. Doing a distribution is a lot of work needing components like installers, package managers, pulling in upstreams regularly, configuration, security etc.
The genesis is a need not being met. It is fascinating what these have been historically ranging from how stable/fast moving they are, how they are built, preferring certain software (eg a particular gui like KDE), wanting a certain kind of community, targeting certain users or uses, and numerous other reasons.
What this means is that as a distro starts it has a reason, a focus and a way to see what work is necessary. The problems start once it becomes successful. It becomes a lot easier to add "one more thing" to the distro such as another package, another configuration option or even good old fashioned feature creep. This will gain more users and make the distro better.
But the larger distribution, more users and wider scope makes the distribution harder. The hardware it is used with, the software it interacts with, changes in upstream and the users all lead to far more work. Every change breaks something, but not changing pieces also breaks due to changes in others. The symptoms of this show up in the bug trackers with increasing numbers of open or abandoned tickets.
Some distros respond by narrowing their audience (eg charging for it, restricting use cases, narrowing hardware and software etc). Others have their developers pull back into the work they are most interested in leaving the other parts languishing. Sometimes politics break out as various parties fight for what they think is important.
Ultimately, people then see their needs not being met causing a whole new cycle of distros.