Active ovaries release estrogen and testosterone into the body. While these hormones are critical for certain physiological functions, they also contribute significantly to a sense of female wellbeing. In menopause, the ovaries decline in function and eventually cease to function. As part of this decline, estrogen and testosterone levels fall. Testosterone is a critical hormone for sexual desire and libido. As levels fall so too does libido. While replacing testosterone in women has risks and is not yet straightforward, there are other lifestyle and psychological measures which can support some measure of increased libido.
2 Reference: https://www.menopause.org.au/images/factsheets/AMS_Will_menopause_affect_my_sex_life.pdf