Prince Philip, Duke of Edinburgh is husband to the Queen of the UK but does not has the title of king. How did that happened? He was just given the title of Duke and was named Prince?
In the year 1701 the british parliament introduced a set of rules on how the british crown is passed from one king to another.
One of these rules is that Spouses don't count as a heir for the crown. They are known as royal consorts and just recieve the title of either prince, or princess.
If you want additional info on how the british monarch is determined, you can watch this short video explaining the 1701 rules.