NSMoWandS hit the nail on the head. Containers are more flexible and dont have to be handled in dedicated trains. The TC's are a pain in the Butt to put together, take apart, and god forbid you have an equipment failure on the road. The time it takes to lift a container out of a car, set it on a chassis and drive it to its final destination cant be any longer that it takes them to put together or take a apart a 150 car roadrailer.
I watched them "set out" a bad order trailer one night a couple of years ago near Danville KY, and the whole process tied up both mainlines of the busy CNO&TP and a road crossing for almost 3 hours. While that may be an extreme example, I am sure its not the first time its happened. Its a lot quicker to set out a flat or well car, and those can be used for other traffic to. The TC's where a good idea, and NS has made some good revenue off of them I am sure, but like anything else...change in inevitable.
I have noticed more and more TC equipment with the rail gear removed, riding on flat cars on 215, 216, and 229 through here in the past year. A far cry from watching the dispatchers "part the red sea" 15 or 20 years ago to let a GP50 and 5 or 10 TC's roll through....back then, they where hottest things on the RR.