What would cause menstrel cycle not to come in young women 21 and younger besides being pregnant?

Variable reasons. Many teenage women have irrguler cycles simply due to the normal maturation of their bodies. Overweight women of all ages are apt to have irregular cycles due to anovulation associated with pcos. Underweight women sometimes fail to have regular cycles because of their body composition (lack of fat). Still other women who are very stressed (mentally or physically) will have irregular cycles.