Given it's difficult to connect up the mains to a flying aircraft (meaning you have to fly with a large volume of weighty water), the lack of rainfall shower is entirely due to water consumption restrictions. Similarly, the lack of a marble shower alcove is again weight restriction (and safety restriction) related.

You can't really expect the full ground luxury shower experience in the air, even most large private jets (apart from the extreme luxury ones) can't pull that off.

While more basic, it still gets the job done with a little modification in shower routine.