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Only Akira Amano knows. However, it may have something to do with the age; Yuni was really young in the future, looking to be around Bluebell's age; she wouldn't be a very good Arcobaleno at her present form's age, as she would have the maturity level of Bluebell, which you saw.

I have reason to believe, it has something to do with Aria. I believe Aria wants the curse to be removed off her daughter so that Yuni can live a longer life and not die at such a young age. Although my theory may be incorrect, it seems to make sense. I hope this helps :)

It is past Yuni. Remember Yamamoto was like "you're so little" or something at one point. 

In the Curse of the Rainbow is the past (or the normal) Yuni, there is a chapter where you can read it.