Why do few models sucseed?

Mainly because the competition is very tough and there are more models than there are available modeling jobs. Because modeling is all about youth, there is a shelf life and unless a person hits supermodel status, they will be retired in favor of a newer, younger person. The turnaround time is very fast for models in general, especially in the fashion industry.

Additionally, the nature of the work does not fall under the category of a career that has security or longevity. Models are independent contractors, which means they do not get health benefits, retirement savings or other benefits that would come with a traditional job, which can make it difficult to make a good living off of.