I can’t speak for everyone, but I prefer hard shell cases for my electronics as it protects things better. (And even if it doesn’t, it gives me better piece of mind.) My backpack is a bottomless pit of stuff. The hard shell, while perhaps taking more room, better protects the keyboard when I’m in a hurry and throw a massive power brick in on top of my keyboard. In a soft sleeve, that may damage the keyboard. Even if not, the power brick pressing down some keys for a week or two straight (say when I’m on vacation) can’t be good for the springs.
At the same time, if I have my small backpack (which neither the sleeve or the hard case would fit into), it’s easier to separately carry the hard case, and again, provides better protection (in my humble opinion). And there might be times all I’m carrying is my keyboard (no laptop etc.)