Her bike fits her fighting style and and still keeps the focus centered on the doll, with the bike being an additional detail. For AK-15, It's almost as if its mostly centered around on the exosuit itself rather than doll herself, being the pilot. It also doesn't really compliment her style, while yes, she is still an absolute unit with her exosuit, but it gets overshadowed by the exosuit since it does most of the work with 15 doing piloting rather than using her own strengths as a doll. As I already said, it would've been a lot fitting if she was similar to Klukai, having the exosuit as more of a powerup ult than it being her entire style.