Why do film studios not release certain old movies? For example, a few of Monroe and Hepburns's work isn't released on DVD. Why?
The answer is most likely one of two possibilities:
- They don't feel that the investment of preparing and producing the DVD will be justified by its sales
- The copyright of various elements of the movie is ambiguous or widely distributed, and resolving that situation legally is cost prohibitive, if it's even completely possible
For the first one, regardless of what it costs to produce the DVD, if they project that only a few hundred or a few thousand people will ever buy it, they may not feel too rushed to get the job done for what amounts to a few thousand bucks to the bottom line. Doesn't mean it will never happen, but that no one has strong incentive to make it happen.
For the second, copyright can be a complicated beast, and any number of people involved in the original project could own copyright interest in the final product (depending on how the contracts were set up originally). That is further complicated by people who have sold their interest or who have died and left it to potentially numerous beneficiaries, the details of which are difficult and expensive and, potentially, intractable to work out. This matters because a work is licensed to be released only in certain media, when a new medium arrives (such as DVD), it needs to be re-licensed for the new medium. If a particular movie suffers from this problem, it's unlikely to ever be released in its original form, what they'll do is identify the portions of the work for which they can't identify or secure the appropriate rights, and somehow remove or replace them. You'll see this most often in TV shows that were current prior to the advent of DVD, where the popular music played as a soundtrack couldn't be licensed, or not at a reasonable cost, and so is replaced by a royalty-free soundtrack instead.
If I had to guess for the particular situation you bring up, the first situation is probably the primary influence behind whether it happens or not.