Somehow it is both a fast lane to fame and a fast lane to obscurity. Again, kind of like testifying against the mafia. After the people walk off stage I picture them being hustled into a van by US Marshals. "Great song. Once we land in Nebraska you will officially be known as Pam Smith and start work in a store that sells only cat toys."
Being promoted by a major TV network can be good for racking up a large following quickly. But that does not seem to help you keep a following, especially when next season the network not only abandons you, but also puts its weight behind a whole new crop of singers.
It's not about the followers who appear only to disappear. It is about the followers who stay with you.