Looking back at it I think I know rhe reason why story stages in 6 weren't available, if you select one and fight in certain ones the game has a noticeable drop, as if it's stuck in the pre rendered story mode frames. Although it's mainly the Colloseum stage ones tho, I think they just never bothered to optimize them for outside modes xD
To hear TN say that means budget was involved again, which in turn leaves to the one outcome problem: Koei Tecmo.