Why are some rishonim referred to with the word "the" (e.g., "The Meiri")?

Question

In common usage, some names have the word "The", others do not. Why?

Why "The Meiri", "The Rambam", "The Ramban", "The Vilna Gaon", "The Semag", but "Rashi", "Ibn Gabirol", "Ibn Ezra", "Rabeinu Tam"?