This is actually only half the problem. Classical music is one of the genres where the profit-sharing model doesn't work very well. Spotify pay out every time a track is listened to (IIRC, this is triggered when you're 30s into the track). This works well for pop music, where the songs are short and they are listened to many times by the typical user. It works less well when the "songs" are 45 minutes long and typically only listened to once (famous works are recorded by multiple orchestras and listeners are likely to check out several and perhaps only ever return to their favorite interpretation).

This means that for many it's still more profitable to sell a small amount of CDs than to be on spotify.

I suspect this will change over the years because there's now a very large overlap between those that still listen to CDs, don't use spotify, and listen to classical music.