because of the drain hose and water valve protrusions.
Personally, I prefer transporting them on the back. The top cover has a rear overhang which offers some protection to the inlet valve and on later models the inlets are recessed anyway.
The drain hose is pain, I prefer to remove it for transport whenever possible. Also, laying on the rear hopefully limits any damage to back panel where it won't be seen once installed.
Just my 2 cents...