kinda the same reason people suggest something like linux mint over slackware, gentoo, arch, etc… mint is easy to install and is preconfigured to be an easy to use user desktop environment. you can configure any other option to be have like that, but they tend to be a bit more “DIY”, which is great if you know what you’re doing!

dedicated NAS OSes will have good software out of the box that make it easy to configure and manage various common disk-related configurations (RAID, SMB, NFS, etc). you can certainly do all this yourself, but it might not have a pretty, unified user interface, or you might have to deal with software that isn’t compatible with some version of a library that’s in your distro of choice… all resolvable things, but they take time to solve: anywhere from installing a package manually to applying a kernel patch and recompiling the kernel to get something to work