Honestly, I think they wanted to weaken the girls category. They got rid of Jade at JH and Amelia in the first live show, leaving Janet (who lacks the mass appeal needed to win), Sophie (who lacked the screen time needed to win), and Misha (who could have stood a chance of winning, had she not been conveniently hampered by bullying allegations that the producers probably scripted). IMO they wanted The Risk to win (the groups category really needs a win in order to justify its existence), and the girls looked like the biggest threat to that.
Of course, this was kind of ruined when The Risk lost support (and members) week on week; after they went, the producers probably decided they'd rather have a girl win than a boy, so they brought Amelia back.