Wizard of Oz rights are held by MGM and Return to Oz was Disney. Besides it being closer to the books, you got to remember that MGM certainly wasn't going to give any more rights to the images than they did. Disney barely got the rights to even use the ruby slippers. And that's all they gave up the rights to.

Wizard of Oz books might be in the public domain but the imagery of the movies is still copyright protected, down to the shade and color of the ruby slippers.