” that only leads the question back to why was it translated before the aruch hashulchan”
Well for one thing it is MUCH shorter! It only covers 1/4 of the Shulchan Aruch. And the author of AHS restates the Shulchan Aruch in his own language, the MB is in the form of a separate commentary.
Dayan Michael Broyde is currently directing a project to translate the Aurch HaShulchan.