If they are re-releases, it is usually from licensing problems.  The
holder of the copyright licenses the re-release of the recording by a
different company only if they agree to limit distribution in defined ways.
The intent is to protect lucrative markets where the CD may be selling
well, or where another recording of the same work may have sales in
conflict.

Also, the artist may have contracts that allow only certain distribution
arrangements.  The artist cannot be forced to compete against their primary
contract label.

Many of these original limited redistribution contracts were developed in
the days of the Musical Heritage Society LP's and Seraphim LPS in the 60's.

I'm sure there are other reasons of which I am unaware.

Ray Bayles