Often this is because they are unofficial remixes or otherwise "grey area" content where the licensing isn't in place to be able to commercially release. If you

Often this is because they are unofficial remixes or otherwise "grey area" content where the licensing isn't in place to be able to commercially release. If you make a track using element from a well known artist, often that track will be allowed to exist in the largely non-commercially exploited environment of soundcloud but as soon as you want to release it for money then you will be hit with a copyright takedown.