Hate to tell you this, but music licenses in Japan can be a pain in the ass! There have been several instances even in Japan in which a TV series, OVA, movie, etc. has dofferent music because of licensing issues. I believe a licensing issue is one of the reasons why there was never an OST released for Gall Force 2 and 3, to use an example. I have a friend that has been buying Japanese CDs, DVDs, LDs, and VHS tapes for about 20 years or so and has run into several instances in which music was missing, so this is not a new problem.
Looking at my CD single of both Zeta Gundam openings, I can see where the licensing of these 2 song could really be the problem. The CD was produced by King Records back in 1988. Gundam soundtracks have usually been produced by Victor. King Records could have easily allowed Bandai to use the songs in the Japanese domestic release, but was unwilling to grant the rights for the US release.