she did straight up pop and had the catchiest songs. Jessica and Christina had some soulful influences. And the closer you are to pop the more succesful you are
See why beyonce and Mariah aren't as succesful as their pop peers. And why Rihanna is more succesful than them Particualry beyonce but not as succesful as gaga or britney or as radio friendly to some extent as Katy
As for Mandy she just has terrible songs

I wanna be with you was her only reasonable single and it was her most succesful single. But too little too late