Why does a woman experience menstruation every month, not every year or two months, for example?

All perfect praise be to Allaah , the Lord of the worlds. I testify that there is none worthy of worship except Allaah  and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Menstruation is decreed by Allaah  for women. It is narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "It is a matter decreed by Allaah  for the daughters of Adam." [Al-Bukhari]

In Al-Mughni, Ibn Qudaamah, may Allaah  have mercy upon him, indicated the rationale behind menstruation saying: "Allaah  created menstruation as a sign that a woman is eligible for bearing a child, and a woman who is not eligible for pregnancy, the rationale is not reflected on her."

As for the question regarding the occurrence of menstruation every month, not every year or two months, this is known only by Allaah, and knowledge of that has no consequences.

Allaah  Knows best.