That's possibly due to legacy reasons. Before snap, every distro did have its own specific instructions. Further more, it even does make sense now: if people saw the list without their specific distro listed due to the fact snap is recommended (Gentoo for example can't run snap by default, so it still lists the original, proper method of installing 
), chances are the user will think "Hey, my distro is unsupported!" and won't click on "snapd" as they won't recognise it as the method they need.
In any case, let's focus this thread on migrating servers and complain about snap in the other thread.