The main thing holding it back is, yes, a lack of interest/awareness because 99.9% of folks don't really care beyond getting "Carl Sagan's Cosmos" in their hands. But a strong secondary factor would be the fact that there was never a commercial release of the broadcast version, which in turn means the only sources that exist for it would be home video recordings. While this is not normally a great barrier, especially for something that was almost certainly very widely recorded (Youtube and archive.org are absolutely overflowing with digitized VHS tapes), the existence of the Cosmos DVD set is what ultimately puts the nails in the coffin.

Just visualize this scenario: "Huh, I have Cosmos recorded on these old VHS tapes. Maybe I should think about digitizing them for posterity. Oh, wait, no: the whole thing was already released on DVD. I guess there's no point." Multiply that by thousands and we have today's situation.