The line is not completely abandoned. There are still locals that service industries on the line. The track between Vera and Plum run is out of service.
I think the main reason for the downgrade was the large number of curves on the route. The nickname for the Cincinnati line was peavine. I think the maximum speed was 40MPH, with many curves down to 30 or less. Once Conrail was split and NS obtained the line from Cincinnati to Columbus, it made more sense to run trains from Portsmouth to Columbus and down to Cincinnati, and vice versa.
However, NS still has all of the Cincinnati Line. If the traffic warranted, NS could reactivate the entire line. It would take a lot of time and money since the signal system was deactivated about ten years ago. NS doesn't hang on to rail lines if they don't think they will be making money from them.