I think talent, combined with a degree of marketability, audience appeal, and a great deal of luck is, always has ben, and always will be, the route to success in popular music.
Leona Lewis certainly has talent and appeal over and above the usual run-of-the-mill winners. The major problem is, the people who watch X-Factor and similar shows, and send these artists' first releases to the top are not people who regularly buy music. Once the impact of weekly TV appearences vanishes, so does the interest and appeal for the mnajority of poeple who buy their singles and albums.
Some are the exception - Girls Aloud being an excellent example - but you need a string of killer singles to keep up the momentum.
Leona Lewis has the talent, she just needs to make sure her profile is directed towards people who will enjoy her material - and that thematerial appeals.