Because this is how the verb is used. The verb "会う" is used like "今朝、ジョンさんに会いました。" In the case of a question, the way the verb is used doesn't change. In the same way "に" should be used. This "に" indicates the object. The only difference between a statement and question is the object. If it's a person's name, you have a statement. If it's the word "誰" you have a question.