possibly for easier assembly.
the Dura Ace FH-7800/7801/7850/7900 all use a retainer.
I'd have to agree. It's easier to assemble with cages. And if you can't do both sides, even doing one side is easier and better for quality control purposes. They also probably looked at their wear-data on the drive side and found that 15 - 20% more bearing contact on the cups/cones on the freehub side would probably help prevent premature pitting since the right side must support both weight load and most of the axle load for pedaling. The left side doesn't structurally need it, so a cage is good enough. But can't the OP swap out the cage with loose after the fact?