The music catalog you see on Spotify is provided and owned by artists/labels. They can choose, for whatever reason, to make their music unavailable from the Spotify catalog. Unfortunately, this is something that Spotify doesn't have control over.
It is best to contact the artist or label via their socials and let them know how badly you want their music back to enjoy them on the go.
I hope that my answer helps you with your burning question, feel free to ask more! - Jordy