Why is a menstruating woman not allowed to pray, knowing that fasting is also forbidden, which negates the reason being ritual impurity?

A menstruating woman is forbidden from praying by scholarly consensus due to the absence of the condition of purity. Evidence for this is found in the Prophet's (peace be upon him) saying: "No prayer is accepted without purification," and in the Almighty's saying: "And they ask you about menstruation. Say, 'It is a discomfort, so keep away from women during menstruation. And do not approach them until they are pure.'"

As for her being forbidden from fasting, some scholars consider it to be without rational meaning, while others – such as Al-Bujairimi – hold that menstruation weakens the body, and fasting weakens it, and the combination of two weakeners causes severe harm.

The Shari'ah has commanded the making up of fasts but not prayers, due to the hardship of making up prayers because of their frequent recurrence, and to achieve the benefit of fasting which occurs only once a year, as explained by Ibn Al-Qayyim. The demand for compliance is a religious matter, whether the wisdom behind it is known or unknown.