they do end up having some kind of a say in what their "sound" is..

aaliyah could've decided to roll with whatever hot-at-the-time producers had made the now more dated/less interesting cuts on One In A Million.
It's not like timbo was katy perry/JT status at that point... he just had a good buzz.
(a buzz that really received its *serious* substantiation beyond gin's "pony" *after* his work with her had come out.)
...but she had the good ear/mindset to roll with the dude who was clearly more progressive.

keyshia could make the effort to reach out to someone doing something a bit more interesting like timbo was at the time, but...
http://www.youtube.com/watch?v=jdLAvOMHpqo

EDIT: the only reasonable excuse is that aaliyah existed in an era where R&B/Hip Hop resided in the center of the pop zeitgeist...and people bought records.
i.e. as much as the genre was lamented back then, in retrospect there was still a lot more leeway to have your cake and eat it too, so to speak.
whereas keyshia has existed mostly in an era where urban music's pop value has been notably maligned, therefore in a business sense she could risk making herself invisible in an already partially commercially dormant genre... but in my eyes that's all the more reason to just go ahead do the hail mary throw... be as left as you wanna be, fuck it.
(but she would have to want to be.)