It's because it would require the sound team to have to specifically code that mount to play that specific track rather than just default to the standard mount theme (The Rider's Boon or Borderless depending if it was classed as a 'flying' mount or not. As it takes extra resources to do this, SE usually are not interested in going back and changing mount music, but it has happened (the magitek armour, the SB Primal mounts). But it is still unlikely.

Also it was my understanding that the Eureka music is from FFXI, which, frankly, does not suit Ozma at all, given Ozma and it's design both came from FFIX, a completely different game.