Since Prince Ranier is the leader of Monaco, why is he not "King"?

Answer 1

Monaco is officially a "Principality", therefore its leader is a Prince. In the same way, England is officially a "Kingdom", so its leader is a King (or Queen).

I suppose if he really wanted to, he could re-name the country to become a Kingdom and take the new title, but its pretty much just a formality thing at this point anyway.