[Mr V]

One reason to put the chains on the inside is to allow for some uprights attached to the outside of the frame rail to create a frame perimeter outside of the wheels and a place to mount the bumper. Depending on your wheel width you can get away with 1x2 or 1x3 for those uprights.

Another benefit is that it reduces the cantilever of the axle by putting the wheels as close to the bearing as possible rather than spacing it out for two sprockets and any spacer required to prevent the chain from rubbing on the frame or wheels.