Whenever they make changes to the program that they are maintaining, which break backwards compatibility, for which an example is replacing X11 with Wayland in the Linux distribution that you may have used for many years, then that forces the users affected by the changes to do potentially a lot of work, in order to find alternatives.

For some special application that you use from time to time, finding an alternative and switching to it may be simple, but when the incompatible changes affect a fundamental system component, which must be used all the time and without which nothing works, e.g. Wayland or systemd, then you must change not some single application, but the entire Linux distribution, and that can be time-consuming, because you may have to learn to do a lot of things in a different way than you are accustomed to.

So obviously, users are not happy about such changes that push work on them without any benefits.

The better Linux distributions may offer their users choices even for such important components like X11 vs. Wayland or OpenRC vs. systemd, for example Gentoo, but the most popular Linux distributions tend to not offer choices for this kind of system components, so when they replace such a component, the users must either accept the change or stop using that Linux distribution, and both choices are bad, because they must adapt their workflow.