My thinking here is safety and replaceability.

You put something on a range expect it to be shot, hence chain that can have links replaced rather than needing to replace an entire cable.
Depending on the cost of the motor and drivewheel it might be an idea to mount it at the shooters line rather than 25m ahead - resulting in a target moving from 50m to 0m rather than 75m to 25m.

The question still remains, any engineers here who can suggest weight of chain, size of drive wheel etc, or would the whole thing sag too much over any worthwhile distance?