MGM only owns the six Monogram Chans released in the "Chanthology"; Warners owns the others.

Warners, when asked about the delay in releasing the Bowery Boys films, has said that it is taking them awhile to find good elements, so I imagine the same may well be true about the Chans. They may have been willing to license BLACK CAMEL to Fox since it was the "odd film out" in their Chan collection.