Models are fairly generic placeholders, attractive men and women who anonymously walk down the runway at a fashion show or stand up for a photo shoot. It's surprisingly unglamorous and isn't particularly steady or financially rewarding.
Supermodels, on the other hand, are a relatively recent innovation, and they are in a totally different (financial) category. The interesting thing is that everyone benefits: the model makes a lot more money, the advertiser gains more credibility from using the known face and the audience gets the frisson of recognition that comes from celebrity. Supermodels aren't necessarily prettier, they're merely more famous, a niche that serves all the parties.
There's a leap between model and supermodel. There isn't really a stable niche for reallygoodmodel and extremelygoodmodel. You are either seen as worth the super premium or you're not. This quantum leap from one state to the other makes it an unpredictable career, one fraught with risk, because you never know when you're going to pop.
You've probably guessed that supermodel status exists in many fields. Stocks, brand names, consultants, doctors, even dog trainers.
The leap must be an intentional one. You don't walk there. You leap.