This happens because the original publisher signed a deal for the commercial music with a limited time.
So when that time expired, any further releases had to remove the music.
Another example of how copyright makes preservation difficult.
I mean, it's obvious: copyright seeks to create a mechanism to compensate creators by creating scarcity. They succeeded.
It turns out archiving those creations effectively is in direct tension with that scarcity. So we made a system to incentivizes creation, and then destruction of the creation. This is visible with the "copyright cliff" in books, but also the incredible difficulty of legally keeping computer programs running over the decades. Copyright's restrictions on derivative works, coupled with the insanely long copyright term, create a sort of digital wasteland, with only those willing to go underground having great collections of old programs.
