Because there are no more egg cells.
a female baby inside the tummy of her mother has approx. 4000 egg cells(I think).
as she grows older(at the age of maturity), the number of egg cells decreases to 2000 or less. it gets lesser and lesser every time the girl menstruates.
when there are no more egg cells, it is when the menstruation stops. or what you call menopause.