Because the menstrual cycle is dependent on your overall health as well as reproductive issues, there are many conditions that can affect it. Delayed periods are usually a hormonal problem, usually because the ovaries are not ovulating. Not ovulating, again, can be caused by a number of issues and some of the more common ones are being overweight, being underweight, thyroid problems, increased prolactin (a hormone that can be abnormally produced), and stress. Of course, the most common reason for not having a period is pregnancy, but you seem to have ruled that out. I would see your doctor to see if any of the other issues I mentioned may be in play.
Other issues involve genetics and without more information or an examination, I can't really address this.