Different goals. Qubes cares very deeply about security. Qubes uses virtualisation to isolate different parts of the system from each other. Immutable distros like Vanilla OS care about providing a reliable base system that can be updated without any problems. They want to achieve this by having fewer moving parts by using readonly container images for all the system software. The user then installs additional user-facing software in their home directory (usually as flatpaks). The additional security is more of a by-product than a central design aspect as it is with Qubes.