It really looks like the looks are chosen to match where the characters moveset inspiration comes from.
You don't really see Perfect Cell pulling off moves from his other forms. Or Frieza doing anything that he did outside of his final form. Vegeta is using moves that he did in Saiyan armor times, and nothing he did in the more plain 1 piece suit that comes with Majin. Do you think we'll currently see 18 do anything that goes against her classic iconic Blue Jean outfit?
This probably helps them say a specific version of a character could take a whole slot later on. Much as Blue and older Saiyans are separate costumes, an Ikari / Super Trunks could have a whole new moveset compared to an Android Saga Trunks. Even Android Saga Trunks has 4 or so variations, for that matter.
And unlike many companies now-a-days, ASW does have a habit of locking certain characters, and even EX versions of base characters, behind story mode completion. Even if alt costumes aren't "Easy" for ASW current style... they're not impossible. We've got 2D sprite fighters that allow you to remove mask, change hair, remove accessories from sprites, and so on. If getting around these limits would allow the to fill the story mode with more battles and characters, I'm sure they could figure some time-saving methods out.