You raise a good question, Dominic. Perhaps the required modifications (new side doors, end doors. possible air brake modifications, as well as the general condition and remaining service life of the Material Handling Cars were not favorable in contrast to buying baggage cars. Nobody but Amtrak knows what the tea leaves said about those things.
I don't think that "cabbage" conversions would be considered good solution to baggage transport except where they can be used as control cars at the rear of a train to avoid turning. Since long distance trains, which carry full baggage cars, are turned at their originating and terminating stations, a cab car is not needed. Cab cars also require Federally mandated inspections, the same as locomotives, plus maintenance of all the control components. $$$$ Again, only Amtrak knows what they projected as cost for that project.
Finally, always remember that Amtrak is not a private sector railroad, and does not operate in the same way. It will always be different, and must endure political factors not considered by private sector railroads.
