In the case of the Cardinal and Crescent, the problem is that Superliner equipment can't go on the NEC due to height restrictions in tunnels.
That's actually the case for all Viewliner-based trains. The tunnels around Baltimore and NYC are far too low to accommodate Superliners, and while the catenary could be raised across the rest of the NEC to accommodate them, it's not worth it if they can't make it through the tunnels.
The nominal height of catenary of the NEC is 21+ feet. They are high enough to accommodate not only Superliners (~16') but even Plate H and Plate K freight cars (20'6"). The clearance problems are at tunnels and select overpasses and stations. Additionally NEC is all high platform, whereas Superliners are incapable of operating from high platform stations, and no one is about to change that on the NEC either.
There is a long standing urban legend that it is very difficult to put catenary on tracks that carry freight. It is just that - an urban legend, and there is no truth to it.
Actually I wish there were more Viewliners on all routes. The accommodation in Viewliners is roomier and less claustrophobic feeling than in the Superliners, specially in the Roomettes. Afterall, there is a reason that Santa-Fe never converted their premier sleeper train - the Super Chief - to Hi-Level. But in current frugal operations it would probably not make sense to complicate operations with multiple types of equipment on a train.