Probably has to do with fleet pricing. Although the cab chassis and cutaway market is relatively small, Ford has dominated it since abandoning unibody for body on frame in the van construction in 1975.
GM going from unibody to body on frame 20 years later should have brought down the build cost of a cab chassis to be more competitive with Ford's, but the RV manufacturers still don't seem to be buying. GM does a lot better on the SRW cab-chassis uses for lighter box vans, and are now the only player in van chassis for ambulances.