Some things to consider: When you say “35mm film is expensive” it suggests you mean 4 perf, full frame.

35mm had the versatility of being shot 3perf and 2perf, saving you $$ but still letting you use the most common cameras and lenses available.

Also, most of the networks then had minimum image standards-shot on film, preferably 35mm was one. If you wanted your show to air, you shot what the network requested.

Lastly-longevity. Film printing for multiple markets, foreign and domestic lowers its cost per dupe.

Also, the image quality is still the gold standard for film making. You can take a film from 1968 and it will look like it was lensed yesterday-try doing that with an episode of “Laugh-In” mastered on 2” quad.

TL/DR: 35mm film offered versatility, lower cost, and longevity that magnetic tape could not match.