Fortinbras is a king and Horatio is not so the statement has more credibility. Fortinbras is be more aware of what it takes to be a "most royal" king than Horatio because he evidently comes from a long line of royalty. We know both his uncle and father were kings and we can assume other relatives had royal blood. Thus, he has watched royal behavior. Horatio, even though he is a loyal friend to Hamlet, has only known Claudius. He was not truly acquainted with Hamlet's father and no other mention is made of Horatio's knowledge of other kings. So, the audience is left believing that Hamlet would have been a good king if he had lived. What better praise can he receive than that from a rival?