If it were single thread on the right, you would never be able to apply enough force to the tiny notches to unscrew it. At least without a tool resembling a giant freewheel remover. On the left, you have nice big flats to stick in a vice, so you can apply more force.

Also, this prevents you from putting the left cup in the right side of the shell, though I'm not sure there's any reason not to.

In an ideal world, LH threaded fixed cups and freewheels would also be double pitch threaded.

BTW the FM left cup has no flats, but the Kingsbridge fixed cup tool will remove them