Via Lawrence Lessig, an interesting Hollywood Reporter article:
Many old TV shows are not being released on DVD because of the cost of obtaining the neccesary rights to the music they contain. While rights to use a song in a film include rights to DVDs etc., the same does not apply to TV series. In some cases shows are being released with the original music dropped or replaced.
This looks like an instance of a wider issue of music being overpriced relative to video. Take any film more than a couple of years old, and you can buy the DVD for less than the CD of the soundtrack album. (As examples, I've just checked Titanic and Fellowship of the Ring on Play.com). How can the soundtrack album be worth more than the film? And how can "Who are you" be worth a significant portion of a series of CSI? Any economist got a view?
Lessig cites this as an example of an "anticommons" - an area where too many parties have the right to prevent a good being used, so it ends up underused. This may be correct (transaction costs are significant), but without an answer to my question above, I'm not sure it's the real explanation.
Update: On reflection, there is a probable reason for the CD / DVD pricing disparity: competition from free TV. Films more than a few years old are likely to be shown on TV, leaving less reason to buy them on DVD. CDs are less subject to this kind of competition - the songs on them get more free-to-listen airplay when they're new than when they're old; if I want to listen to the Titanic songs, I've pretty much got to buy the CDs, but if I want to watch the film I just need wait a few months until it's next on air.
It turns out this has little relevance to the TV-show music issue, so it leaves the "anticommons" theory as a strong contender.