Because in Europe, it is not the custom to give the husband of a queen regnant-- a woman who reigns in her own right -- the title of king.
Whereas the title "queen consort," for the wife of a king, is very old, queens regnant came along very late in the history of monarchy. When they did, there was concern about what to call their husbands. The fear was that any kingly title -- even just "king consort" -- would cause the husband to be perceived as the real monarch. After all, for centuries, a man who married the heiress to a throne or a great estate often just took over that throne or estate by law -- called jure uxoris -- because people didn't want to accept that women could reign in their own right.
Once they did accept this, the custom arose that the husband would be called simply "prince" or "duke," so as to make it clear who the real ruler was.
Queen Elizabeth is currently the only queen regnant with a husband called prince or duke, but until recently, Queen Margrethe of Denmark's husband was a prince, as was the husband of Queen Beatrix of the Netherlands. (Both men are now dead.) When Crown Princess Victoria of Sweden becomes her country's queen regnant, her husband, Prince Daniel, will remain a prince. He will not become a king.