Neither format is better than the other. Often times a much more effective picture can be created in the mind of your prospect, than just the blatant image itself recorded on video cassette. Creativity in this area goes a long way. Production costs aside, an audio cassette can be manufactured for less than the cost of first-class postage. This makes it a good value.
Audio cassettes are consumer accessible and very familiar.
The advertising dollar has seen an ever decreasing return for its investment. Clearly, there is a need find a way to spend the bulk of the advertising dollar on converting dollar on converting true prospects into buyers. Imagine customers contacting you requesting your advertising. Asking you to sell them. This is not the modern miracle it might seem.