Medieval and therefore monarchical structures are highly gender segregated. For almost all monarchies, if the first child is a daughter, she will be passed over in favor of the first-born son, even if he is younger. This law was just abolished in the UK in 2015, so the gender slant in monarchies is incredibly ancient and powerful. In feudal systems, the woman can take the titles and honors of her husband because he is considered to be of a higher rank in the system while a husband can generally not take the wife's titles because if he held them, he would outrank her, and since she is the actual holder of the title that can't happen. So, in monarchies, the king is always the absolute highest rank. So if a king's wife is queen, she falls under his ranking and the system holds up but if the ruler is a queen, if the husband was called king he would outrank his wife even though she is the legal monarch, so the titles wouldn't hold up. The person that holds the power needs the highest title and since king outranks queen, kings can call their wife queen but queens cannot call their husbands kings. However, you can do that if the couple shares the political power (like William and Mary in the UK) and they can be called king and queen because they are each rulers in their own right (like Ferdinand and Isabella of Columbus fame).