One reason is that currently distributors have to jump through hoops to get classical music on Apple Music. Pop music is required to provide the usual info you'd expect (artist, album name, publisher, date). But for classical music, Apple wants a myriad of additional details that the publisher/distributor needs to fill in, and many don't want to go through all that effort for the small classical audience that Apple Music has. Most other music streaming services don't require that level of detail for uploading classical music to their services.
