Thats quite often the case, especially with things like phantoms, or f65's, PL lenses or even reds, which do somethings exceptionally well, but are of limited interest for other applications. You wouldn't want to do a live broadcast from a RED for example.
Also many productions are set up as limited companies which only trade temporarily, expensive assets with high potential losses are less appealing than hiring a set up for a well organised short period of time.
For example, you can't buy Panavision gear, productions hire in as required. No maintainence issues, no getting pithed off when a better camera comes out etc.
The 1C has the 4k benefit over the c100 & c300, but thats only important if you are shooting 4k.
I think the 1C is also an exercise in seeing whats possible from larger resolution sensors, as the c100/300/500 have lower resolution but also reduced moire etc, reduced jello etc.
I don't think a 1Dc would be the first choice of a film-maker, and lets not forget, the c100/300/500 also have stills capability, but they wouldn't be the first choice of a photographer.
I think the nail was hit on the head, the market for the 1Dc may well be staff photographers at agency, who have to gather photo and video, to different standards.