Why is amenorrhea common among adolescents?

In part, obesity. Certainly amenorrhea among adolescents is an issue that is way underestimated simply because most teenage girls of menarchal age do not admit or seem to be bothered by lack of menses. In my practice, the majority of adolescents have amenorrhea of secondary type (meaning that they have had a period or two in the past 6 months) that is due to obesity +/- polycystic ovaries syndrome (pcos).
The cycles forming! It is very common to experience amenorrhea in early adolescent years because the co-ordination of cycle is still developing. An emotional change such as moving out of home often is associated with it.