What is the main reason why some client’s STOP using personal trainers ?

As the others have stated: results and money.

From a results stand point it could be both results achieved or a lack of having achieved results.

A third reason that I’ve seen a lot of recently is moving due to a job. With the economy being what it is, a lot of people have had to change jobs or more altogether to get a new job. The trainer they were seeing may not be close to their job or home any longer.