It takes more than zero dollars to prepare media for a new format, like streaming.
Many old series did not have contracts that covered streaming, since it didn’t really exist at the time.
Many old series had music licensed for a specific format, and they would need to renegotiate those contracts for the new medium.
That is why some series have made it to streaming but with replacement music or are only available to buy but not be included in netflix or hulu as part of a subscription. Others haven’t made it because the company either thinks they won’t make their money back or they are working on something that they expect will make more money first.
The making money back, contracts, and music applies to sales too.
The reason they don’t have it for sale is because they like money.