Syco and other labels involved in these tv shows favourites are those with the biggest existing fan bases. They want as bigger return as possible for as little effort possible. Acts like 1D, Little Mix, Fifth Harmony, Union J etc. all got signed up with large pre existing fanbases meaning anything they released was always bound to sell pretty well at least from Day 1. They were safe bets. All the discarded acts, like Diamond, will rightly or wrongly have been deemed an unjustifiable gamble.
You can see why Cowell is so keen on groups at the moment.