Is it down to a rights issue? It's just that I notice that some do appear on there and others dont. Like From a Whisper to a Scream by Jim Manzie, or Salem's Lot and Gremlins Expanded etc.
Basically, if the Lt Edition soundtrack labels put more stuff on Spotify I'd be over the moon. But I'm guessing they would majorly lose out money wise. Is that why there aren't more 'rare' soundtracks on there?
With very few exceptions, the specialty labels don't have streaming or download rights. They negotiate to put out the CDs alone, and the tapes go back to the studios to do with as they wish.
The larger question is whether these streaming services can survive. While (some) fans love them, the services are bleeding money and the artists and labels are making precious little. Maybe in five years, they'll be hugely profitable, but it's a little hard to see that from here.