There’s a pretty simple reason. It’s that developers don’t have to spend the time to package for every single distro. I know I wouldn’t, I’d just focus on packaging for the distro that I use and flatpak. Having flatpak also means that some less known distros start with a big amount of apps available from the get go with flatpak.
I’m pretty satisfied with Fluffy but the clients do still need a lot of work indeed.