Spotify wants all the world's music, and they're working hard on making this happen for you. However, sometimes agreements can't be reached with the artist or label or a change may happen in music ownership.
Spotify adds thousands of albums and artists are added every day. If you can't find an artist you're looking for, they may appear (or reappear if removed) soon.
Garth is a big time artist's rights activist. He was one of the major figures speaking out against Napster back in the day. He loathes YouTube (called it "the devil"). Even though he released singles, he's also blathered about "album integrity." He won't work with Apple (still not on iTunes), and had to have his arm twisted to even set up his own exclusive digital download store.
Obviously no fan of streaming. His refusal to accept the realities of the industry and his clinging to old models make the late Prince look like a futuristic visionary about the role of technology in content distribution.
In short, Garth Brooks will probably be the last holdout. After Def Leppard and Peter Gabriel are established firmly on Spotify, Garth will still be clinging to fighting the quixotic fight and begging you to buy CDs. That's how extreme and isolated his position is.
Yeah, they knew the exclusivity would appeal to him and offered a sweet deal. Exclusivity and balkanization have to end eventually. Tay-Tay is largely to blame for the waning, but continuing scourge of exclusivity. She's come around, but the damage was done. Whatever else you say about Daniel Ek, you have to admire his refusal to give in to the pressure of offering exclusives. He may save the music industry yet (even if it remains maddening that he and his minions still refuse to give us specific explanations for song removals and instead offer up silly suggestions to "contact the label").