Just cuz. Macross 7, incorporates lots of music from the entire series. It had music from DYRL?, M+, MII and it's own music. Perhaps Kawamori was paying some tribute to MII, but maybe, he was adding a story flavor. The music style has changed with every Macross series. Maybe it's a fun idea to incorporate music from the rest of the series just to say, well, not everyone likes the same kind of music. Some people like idol-pop, rock, etc. And M7 did exactly that. The majority of it was rock, but Kawamori threw in some other music to say, not everybody on M7 listens to just Fire Bomber.