(This is more for OP and other readers than the author of the comment I’m replying to)

A classic example being WKRP in Cincinnati which was a relatively low budget sitcom when it was produced. It was shot on videotape instead of film and took advantage of a special licensing rate for music when added to videotaped programs, which let them include a lot more contemporary rock music that would’ve aired on a rock station in the late ’70s. However, the licenses had a limited time allowance so while the show was originally in syndication with that music, by the ’90s it was replaced with similar sounding stock music. Early releases of the show for home media also didn’t have the original songs. Shout Factory put together a box set of the show and went back to the music owners to try to form new licensing deals, but even they couldn’t clear every song.

Music isn’t the only factor; similar issues pop up with all sorts of rights issues and royalties. When shows were made in the ’50s no one really had the idea of reruns and syndication. Before the ’80s there was no real idea of home viewing, and even then in the days of VHS tapes the idea of putting an entire show on tape for home use was pretty out there. Only fitting 1-4 episodes on a tape meant a season alone might take up a whole bookshelf, never mind a full series. It really wasn’t until the 2000s that there was a normal expectation that a show for broadcast would also go into syndication and be sold/rented to home viewers. So a lot of contracts with actors, writers, directors, etc. didn’t cover how royalties would be paid on these newer releases. Sometimes those rights have been sold in the interim as well, so it requires a legal team researching what rights need to be secured and who currently owns them to make sure all the payments are planned. Get it wrong and a rights-holder can sue and might end up taking away all your profit, even making the venture lose money. If you’re going to release an old show, you need to be confident that there’s enough of an audience willing to pay that you can cover all those costs and still make a profit, not to mention the costs of preparing the program to a format suitable for sale/streaming.

Of course, once those copyrights expire, some of those cost concerns go away. We’re only just starting to reach that point with films (anybody want to watch Steamboat Willie?), so in another 30 years or so we’ll probably start seeing more old TV shows. If they’ve survived, of course.