A priest who gave a series of talks on the history of the Mass, for my OCDS group, explained that the practice of the priest with his back to the people began back in the early days, after Constantine gave public buildings to the Church.
The buildings were formerly official goverment buildings, rectangular in shape, where the residing figurehead sat at the front and the other officials sat on either side, with the public in the pews. It was typical of a kings court. So the thinking in the west was, Christ being the King at Mass, resides at the front on the altar. However, this was incorrect, because the priest is Christ on the altar and at the Last Supper, Christ faced the Apostles. In the East, the church buildings were round, where the priest faced the people, who sat in a semi-circle around the altar, which would have been closer to the events of the Last Supper.
Anyway, this is the way I remember it, and I’m sure I’ll be repremanded.
Jim