First of all, not all religious life is monastic–in fact, only a minority of religious are monastic.
Second, a person’s entry into religious life is a process, and not a single event or decision. Both the community and the individual enter into discernment. A person may enter with the best of intentions, and then decide that it doesn’t “fit.” Sometimes the community makes a decision that a person is called to another way of life. And coming to such awareness takes time. That is why formation is several years long, regardless of the form of religious life.
As for why individuals might “decide” it was not where they were called to be, I think there are as many reasons as there are people. Just as people are called (vocare) to their way of life as individuals, so their reasons, and their journey are also highly particular.
