A copyright owner has the exclusive right to produce, or license others to produce, derivative works based on their original work. Others cannot just decide to market products linked to a copyrighted work without the permission of the copyright owner. Examples of such derivative works include such things as sequels, translations, and musical adaptations.
The links between Dark Side of the Moon (1973) and The Wizard of Oz (1939) are very ambiguous at first glance. Most of the similarities can be easily written off as coincidence or the overuse of fanciful imagination. So, even though many people seem to identify over 100 "matches" and eerie synchronicities, these "matches" alone are not enough to prove intentional alignment by Pink Floyd.
However, if Pink Floyd were to admit intentional synchronization with the movie, that might legally make Dark Side of the Moon an Oz-derived work. It might be seen as a musical adaptation of the film. In that case, Pink Floyd might be liable for millions in damages for selling the album as an accessory to the movie without permission from MGM (the movie is now owned by Warner Brothers).
A motive for denying intentional synchronization might be to avoid becoming vulnerable to an intellectual property lawsuit if there were any intent at all.