Freemartinism: Rare condition that makes heifer twin useless

In rare occurrences, sometimes heifers give birth to twins. If the twins are both female, this is like a double blessing. If both are males, this is still good. But if the twins are heterosexual - male and female, the male is good but the female may be worthless, as it will suffer a condition known as freemartin. But why? 

A cow breastfeeding its twin calves in Mombasa County. A female born with a male twin suffers a condition called freemartinism. [Kelvin Karani, Standard]