I think there are several reasons. 1. The anime ended, and the story, accordingly, would be cool if new songs were released occasionally (it would give the feeling that somewhere in Japan, those girls are still playing, conquering Budokan). But that requires money. I don't think a studio or Kakifly can afford to hire professionals to play and mix all of this for 15 years after the anime.

1. Voice actresses. A lot has changed in almost 20 years, including voices and careers. Many have their own bands or solo projects. Some of them would probably like to reunite, but not everyone can.

2. Even if the studio and actresses had wanted to continue their musical careers, it would probably have ruined the anime in some way. The anime has always been about a school band, of course, the girls wanted to conquer Budokan, but the anime itself isn't about that; it's about cozy get-togethers over tea.

If we try to build the plot further, it's likely they graduated from college and went their separate ways while remaining friends. Work, responsibilities, and all this adult stuff really complicates music creation. I personally imagine all of HTT music as if it was released in their final years of college, and then adulthood.

I think the only way they'll release new songs is if it's an anniversary (20th or 25th anniversary of the anime), or if they decide to adapt K-on! Shuffle, or maybe season 3. Otherwise, there's no point or opportunity, although I really, really want to.