As a pretty regular Linux user (I am using it on my laptop as I write this, and used to support Linux based equipment), I like to try out different distros from time to time; my current preference in Zorin OS 7. Something that is not often talked about is the fact that many Linux distros - the smaller or "speciality" ones in particular - are often run by a single person, or maybe a couple of people. This fits with the concept of Linux being a sort of "hobby" product, notwithstanding the presence of big players like Red Hat and Ubuntu.
As you can imagine though, after a few years slogging away to bring out useful products for basically no money (and sometimes absorbing a lot of criticism), individuals my just get tired, or their circumstances change - and the distro typically fizzles and dies due to lack of updates. This is especially true if it truly was the project of a single person.
Such is the case with UK-based Solus OS, which crumbled just a few days after announcing a collaboration effort another product. It appears that the driving force behind Solus was offered a dream job, and had to make a hard choice. You can read about it here, from one of the people that was closely involved with the project.
But, on the other hand, new distributions will crop up too - it's almost like a big gardening project; some plants will grow and flourish, and some will wither and die. Existing projects may branch off into something else, and succeed or fail in a different area. In that sense Linux is pretty organic, and in some ways we are just seeing the "circle of life" in operation.