Putting the chain inside of the chassis give you a safeguard for the chain as GeeTwo said, even though it's not inside the tube. WCD was developed before bumpers and other robots could get tangled/break the chain. Also, there wasn't OTS FRC gearboxes then so just about everything in the gearbox was custom. Teams that developed WCD also were fairly strong in their machining capabilities, so live shafts weren't as big of an issue.
Early WCDs (with six wheels), didn't have as small sprockets. There were cutouts in the electrical boards for the chain runs.
You can make it out here. Tensioning was also solved with eye shaped bearing blocks and cams.