Since no one has actually attempted to answer your question yet I'll give my guess as to why: I think Apple generally invested way too little time and developer talent into the development of Apple Music. I think this is evident in many ways when you look at the Music apps, _especially_ the macOS one. Since the initial release of Apple Music very little has happened to the apps, leading me to the conclusion that they're not spending time to improve them either.
Recall that Apple bought Beats to acquire the music streaming platform that Beats was developing with the purpose of turning it into Apple Music. To me this always seemed like a bit of a weird move since Apple could obviously build their own music streaming service without acquiring a company to do so. My guess is that someone at Apple who's more of a business person than a product person wanted Apple to have a Spotify competitor ASAP and saw the acquisition of Beats as a quick way to get there (with means that a business person can carry out: acquisitions). The acquisition happened, Beat's tech was turned into Apple Music (probably with time to market as the key metric that management cared about), Apple Music was released. Then the business person was happy, called it a day, and moved on.
Had Apple put their own focus and top talent into Apple Music the way they did with iTunes and iTunes Store back when iPod was their main cash cow, then I think Apple Music would have been very different. But alas, "music streaming" is probably just a check mark in someones list of services that Apple should have, and it's already checked so the job is done :)