I think the main reason is experience on a distro with a crappy package manager that can easily result in a damaged or even unbootable system. Some of the most popular and even reportedly “stable” distros fall into this category.
You can think of an immutable distro as a system that treats the entire core OS as a single package. As long as you have that installed, it will boot.


I would agree with you. That said, bad experiences with things like PPA’s are one the reasons people reach for immutable distros.
I think as soon as you start adding PPA’s to your system, you should consider a distro with a larger repository. But that is just my opinion.