It's because, before TV on DVD became a major thing in the mid-00s, TV shows would get limited licenses rather than perpetual licenses as a way to keep costs do

It's because, before TV on DVD became a major thing in the mid-00s, TV shows would get limited licenses rather than perpetual licenses as a way to keep costs down. Most episodes would air once or twice (its premiere, and a summer re-run), and if a show hit its 100th episode (at which point it became feasible to license it for syndication), the studio would renegotiate music licenses (and some popular shows never entered syndication because of a failure to renegotiate these licenses, while others, like The Wonder Years, entered syndication with radically altered soundtracks). Once TV shows started to release on DVD in a big way, and then digitally a few years later, it became immediately obvious that shows would need longer music licenses, and so the issue has been solved for newer works.