Are you kidding me? PBS consistently turns out extremely high quality programming. Cosmos may have been the best mini-series in television history, and it wouldn't have gotten even a sniff at any of the so-called "major" networks. In more recent years, shows like NOVA and any one of the several documentaries by Ken Burns are more than keeping up with the Big Three. I think the only network on TV that rivals PBS in its quality of original programming is HBO. Now seriously, are you actually going to say that "Friends" is a better show than "Cosmos?"
So, they could've tried to sell a shortened version of "Cosmos" at the movie theatre, as well as on reel-to-reel film (being 1980 an' all) for interested school districts and other parties.