Probably because it shaves more than 15 minutes? You can't take the Amtrack from Chicago to Columbus because Columbus is currently not served by trains. You can take a bus, but that will probably take you about 8 hours. If you drive fast, you can drive yourself in a little over 6, but then you're also stuck with a car in Chicago, which is a huge pain in the butt.
Under this proposal, the trip is 4 hours, and you'd be easily plugged into Chicago's subway system.
People also commute into the city daily not far from Plymouth, so this could easily be quite the boon for folks living in Northern Indiana. Daily vehicle traffic the south shore highways suuuuucks