…why cannot it be ported to Windows / Linux to offer a more performant Ruby(like) language for backend development ?
IMO: This is more or less the idea behind Rubinius. There’s also Crystal which takes a different approach. Neither of these have really caught on compared to Ruby due to the limited number of problems which require more performance at the compiled code level.
RM, on the other hand, mainly acts as a bridge to allow Ruby development on these platforms which would otherwise be missing a Ruby solution. The fact that it is fast is just a bonus.