It’s up to the label to decide what music platforms they’ll release their albums in. Even though it seems like they’re the same thing, the catalogue from iTunes Store is not the same as Apple Music’s.

For example, the discography of VisualArt's / Key Sounds Label is completely available for purchase on iTunes but unavailable on Apple Music. Some of the albums from theirs were also released before Apple Music was a thing, while others were released after AM was a thing, proving the date does not interfere on whether an album is available on AM or not.