TPTB doesn't really cares about who wins, who is the best singer. They want people that can make them look good. And that will be with commercial success.
Fact is most of these people will be forgotten in 2 weeks when the show is over. It happens every season. Social media stats don't mean a whole lot unless they translate into real fans who are willingly to spend money on original music, see their favorite in concert.
While I agree with you that Abi would sound really good singing Turning Tables, I'd really like to hear her cover Hello. And if Rolling in the Deep is going to be sung, I'd really like to hear a male cover version of it. Do you think Will or Jack could do an adequate job covering it?