There are only three companies that control distribution of almost all popular music. (Universal, Sony and Warner) Once you get outside of artists and genres published by these four (e.g. smaller foreign artists, very small niche genres, small indie artists) streaming options decrease.
I suspect Apple snapped up some very long rights early on which means they’re not going to bother trying to compete directly as they’d only have their own songs.
Same way that a bunch of studios waited for their Netflix/Prime contracts to wind down before opening their own streaming service.
