Why won't the days a woman is menstruating in Ramadhaan count?

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 is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is forbidden for a woman in menstruation or in her confinement period to fast or to pray. If she prays while in this situation she is sinful and it will not be accepted from her. However, it is obligatory on her at the end of her menses or confinement period to make up the fasting but not the prayer. The evidence is the saying of Aa'ishah : "We used to be in such a situation, and we were ordered to make up the fasting but not the prayer." [Muslim] Allaah has legislated this for a woman as a sign of mercy to her. Some scholars sought the wisdom behind  this reason and concluded that menstruation weakens the body of woman and so does fasting, and if both are joined together then there is a harm for the woman and the rule is that one should not harm himself and should not cause harm to others. Indeed the Islaamic jurisprudence considers the protection and safety of our body, and so such legislation is indeed appropriate.

Allaah knows best.