I think it goes back to traditional gender roles in which the woman was assumed to be subservient to the man unless her rank was higher (king is higher than queen, but queen is higher than prince), and especially when a queen regnant married a foreigner, there was always a lot of fear about whether the queen's husband, being the male half of the couple, would predominate and make the country just another colony of whatever country he'd come from. (Bloody Mary and the politics surrounding her marriage are a good example of people worrying about this.) So if the king marries some woman, it's "safe" to call her queen, because the king is still in charge. But if the reigning queen marries some man, he gets a title that places him lower than the queen, to counterbalance the fact that, as the man, he was assumed to be the dominant partner. Or at least that's what I think the thought process was.