Are they so different that it's justified to have so many different distributions? So far I guess that different package manager are the reason that divides the linux community. One may be on KDE and one on GNOME but they can use each other's packages but usually you are bound to one manager
I wouldn't put it that way. In the case of Rust, it seems everyone wants to have a stable ABI for a number of reasons (e.g. making dynamic linking possible without FFI), but the core developers feel like the ABI is still too unstable to commit to anything.
In my experience a lot of Rust developers love the lack of shared libraries and bundling everything, viewing it as a huge win. Maybe someday it will support it but I feel it will be less commonly relied on.
I've seen that sentiment but I think it's more a matter of people making excuses for Rust and not wanting to admit that it has any shortcomings compared to C++.
It's the same mentality that leads C++ developers to defend things like header files.