Sorry for the late response 🙂 It could be because of licensing agreements with the labels/artists. Because of those agreements, content varies from region to region. This means that while a certain song is available in your country, someone else who lives in a country that the label or artist hasn't made it available in, won't be able to listen to it. A useful tool to see if that's the case is by entering your song's detailed here.
Sometimes even, agreements can’t be reached with the artist or label, or a change may happen in music ownership. If you can't find an artist/song you’re looking for, they may appear (or reappear) soon.
In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify.