Because a king ranks higher than a queen. If a female monarch succeeds to the throne in her own right, declaring her husband king would undermine her sovereignty.

It’s not quite right to say that a king‘s wife is a queen. At least in Great Britain she becomes Queen Consort to signify that she is not by blood heir to the throne. Similarly, Victoria didn’t want to make her husband King but King Consort. The parliament refused to style a foreign prince King Anything, especially since one major change already occurred upon their marriage: the ruling house changed. Victoria was an offspring of the House of Hanover. However, the man brings the family name into a marriage even if his wife is the rightful queen. So, the ruling house of the UK from then on was Saxe-Coburg Gotha (and still is today although the name was changed to Windsor in 1917 because of anti-German sentiments; also, Prince Philip‘s name, Mountbatten, was later added as a family name). In short, styling Albert King (Consort) would have signalled his and his family’s rule over the UK of GB and Ireland when in fact Victoria was the sole sovereign.

It’s rare that a woman inherits the throne. Kings always wanted their bloodline to be furthered by sons. So, even when a royal couple had many daughters and only their, say, fifth-born was male, he would succeed to the throne (they have changed that in many modern monarchies including the UK). This male-centric idea of succession led to the perception of female rulers being the lesser option. Therefore, her status had to be protected and the rightfulness of her rule highlighted. Making her husband king would have done the opposite.