I have a 14 year old girl who hasn't started developing breast and hasn't started her period. Why is that?

Hard to say. Any girl who has not had at least nipple/breast development by age 14 is considered delayed and deserves a full evaluation. Menses is usually around 2 years after early breast development. Your primary doc could start some testing while they arrange for her to see a pediatric endocrinologist.
Menarche. The age of thelarche (breast development) and menarche (onset of ovulation and periods) has dropped in this country to around age 10 or so. However, there are a variety of factors that cause the onset including genetic, body fat, diet, etc. If you are really worried, see your family doctor. However, menarch isn't considered late until age 16.