If you're a fan of Regional at Best, the 2011 album from Twenty One Pilots, you might be wondering why it's not on Spotify. The answer is complicated, but it boils down to two main reasons: first, because of the way Spotify licenses music, and second, because of the way that album was released.
When an artist signs up with Spotify, they agree to let the platform stream their music in exchange for a percentage of the ad revenue generated. For most artists, this is a good deal - but it doesn't work out so well for regional acts like Twenty One Pilots. Because they don't have much of a following outside of their home region (in this case, Ohio), they don't generate enough ad revenue to make it worth Spotify's while to license their music.
The other reason Regional at Best isn't on Spotify has to do with how the album was released. When Twenty One Pilots self-released the album back in 2011, they didn't put it up on major digital platforms like iTunes or Amazon Music. As a result, there's no official digital version of the album for Spotify to license - and without that official version, they can't legally add it to their platform.
So if you want to listen to Regional at Best on Spotify, your best bet is probably just to find someone who's already uploaded it illegally. Sorry!