It's the whole "Simon Says" issue.
Steve Brookstein had a number one album, with little promo, but was dropped as he wanted to write his second album. Cowell wanted him to do another cheap covers album.
His main rivals, G4, were given much more promo & treated as the "real winners".
James Arthur did well until he was dropped. The show already had Ella Henderson lined up to be the star. Rumour has it that the massive amount of PR spent, buying up tv & radio slots, to into turn her into a star, means the album is unlikely to ever become profitable.
The same thing seems to be happening with Fleur East. Her current single is in several advertising campaigns.
Sam Bailey was dropped when she started wanting more say in the direction of her music.
Ben Haenow will be dropped soon enough. His promo relative to Fleur's is derisory.
Matt Cardle was the biggest selling winner. He was dropped after wanting more say in his second album. His manager mentioned that the media seemed to blacklist him after he split with Syco.
This resonates with Steve Brookstein's "we'll bury you" comments from Max Clifford.
Olly Murs & JLS were pretty much given access to free mass advertising via TXF. Olly is even performing on the show again this weekend.
One Direction had the biggest investment any group has probably ever had to make them a thing. When they first hit America, drama students were paid to be "screaming Directioners" and make it look like they were really popular. These images were beamed across the world by the paid for media to get impressionable people to jump on board the hysteria, like the last minutes of a stock market bubble that no one wants to miss out on.
Little Mix have again been massively hyped. The American media weren't interested in them until the Zayn/Perrie "relationship" happened. They had slots in shows such as Glee for their music.
Despite all this, it's probably well to look at the real statistics. One Direction don't sell as well as people think. Even I was surprised at how low some of their chart positions were.
If Wiki is correct, More than this only reached number 86, Midnight Memories 39, and You & I only 19.
Is it possible that, despite all the hype, many "fans" are really just celeb chasers who don't buy the music?
https://en.wikipedia.org/wiki/One_Direction_discography