The reason is that the artist or their music label has likely decided to remove it from Spotify.
The availability of music on Spotify is up to the artist and their music label.
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. Sometimes the music is only available in certain countries, while access is blocked to others. Sometimes the album or song will be unavailable temporary, and come back again later.
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.
You could try emailing his managers directly and asking them why. It's possible Mahone's label yanked his music without telling him. It wouldn't be the first time an artist's label has done something shady.