There are a couple of possible reasons for this:
1. It never aired: It's possible that the film was produced but never actually broadcast.
2. Limited release: It may have been shown in a limited release, perhaps only in certain theaters or on a specific network.
However, there is a 1949 film version of "The Red Pony" that you may be thinking of. This film did have a wide theatrical release.
To be sure, it would be helpful if you could provide more context:
* Where did you hear about the 1973 TV version of "The Red Pony"?
* Is there any other information you know about the production?
With more details, I might be able to help you track down more information about the film.