It seems her and Cowell never always saw eye to eye resulting in creative differences. When her previous albums underperformed, it meant her popularity had deteriorated and by then, she no longer became Syco's top priority. One Direction came along, stormed the world and made a shitload of money for the label. This gave Cowell further incentive to invest in them greatly, while Leona was pushed to the side.
Moving on was the best decision. There is simply no point staying with a label that no longer has her best interests. It will be interesting to see if Cowell will still allow her to promote her new music on the main X Factor or Britain's Got Talent show, whether she will be downgraded to perform on their ITV2 spin-off shows or if she will be invited to perform at all.