There's always more to be said (in theory), but I have a past answer to a very similar question that I'll paste below.

Monarchies have not switched from having a king and a queen to having just one or the other, even in the UK. What you're seeing is the vestiges of sexism: the wife was historically inferior to her husband, and so the role of queen was automatically assumed to be inherently inferior to that of king. Therefore, when a man was king it was no problem to have his wife crowned as "queen consort", but for a ruling queen to have her husband crowned as "king consort" would have added a certain amount of confusion as to who was in charge. Therefore a ruling British queen has her husband take the title of "prince consort".

The first time this issue had to be grappled with was in the reign of Mary Tudor, who married Philip II of Spain. He would actually be a king in his home country, which led to consternation in England - would he expect to rule both England and Spain, as husbands had dominion over their wives? Even earlier in her life, this had come up - as a child, Henry VIII had wanted to marry her to James V of Scotland, working on the assumption that they would unite Great Britain and that Mary would not actually be a ruling queen, just a consort. Through their young adulthoods, she and her sister Elizabeth were never married off by their brother Edward VI because it was simply too difficult to figure out how to deal with the issue. In the end, Mary had a marriage contract drawn up that gave him the title of king just for the term of the marriage and gave him very little actual political power. (And of course Elizabeth would side-step it altogether by not getting married.)

Queen Anne, the next queen who ruled alone, married when it seemed unlikely that she would take the throne, as the co-rulers, William and Mary, might have had children. As sister of the queen, it was appropriate for her to marry the also-unlikely-to-rule brother of a king, Prince George of Denmark, which made it easy for him to be given a back seat when she did inherit the crown. He had no interest in pressing a claim to be superior to her by virtue of having married her, and was given the title of "prince consort", a tradition which survives to this day.

In a western-European context (which is perhaps the only one where this question is valid without using different terms), I think the issue here is that although there's a perception queen is the feminine form of king, this is not actually the case. King derives from a root related to kin, and effectively implies head of a family/community. Queen derives from a word meaning wife, and bears no original connotation of rulership. Rather a queen was a special form of wife, whose sons were throne-worthy and therefore had a better claim to the throne.

The background to this is the fact that no early-medieval European kingdom had clearly-established rules of succession, so that kingship was not necessarily limited to one family, and even within a family primogeniture was not the norm (e.g. Alfred the Great succeeded his brother Æthelræd despite the fact Æthelræd had two sons, one of whom in turn unsuccessfully challenged the succession of Alfred's son Edward). In an attempt to give themselves higher status kings took to being anointed, that is following the Biblical model they were through a religious ceremony made into a special class of person who were protected by God, and who were therefore uniquely fit to rule. But as anointment only worked on the anointed person, it was of limited value for ensuring the succession of sons (both Offa of Mercia and Charlemagne anointed their son as king besides them in their own lifetime, but as Charlemagne's son Charles predeceased him and Offa's son Ecgfrith died less than a year after him this may not have caught on as an idea).

The solution to this was to produce sons from a union of anointed parents, so anointing the king's wife and giving her a particular status would in effect make their sons particularly worthy to be king, whilst making the claims of children of other partners, other members of the royal family and outsiders weaker (as anyone who has read any medieval history might note, this wasn't a foolproof scheme). In English the word queen became attached to these favoured wives, but not other royal consorts (for various reasons not all royal wives were made queens).

It became increasingly normalised for kings to be married to queens, so that by the death of Henry I of England there was no question that his many recognised sons from women who were not his queen could succeed him, and which meant that there was an expectation every king would have his queen.

It was only in the early-modern period when circumstances required the development of a terminology in English for a female monarch. By that point both in English and Scots the word (ancestral to) queen was fixed as the lesser half of the ruling couple, and was a title often used by queens acting as regent when their husbands were away from the government or when their sons were minors. So it was easy enough to have a queen ruling, as this was not totally new, even if a queen ruling in her own right was.

A really-good (if slightly old) background to the origins of western-European queenship is Pauline Stafford, Queens, concubines and dowagers.