In fanfiction, Bella might be the Volturi's princess because of a misinterpreted prophecy. The Volturi could believe that Bella is the key to something great, like ruling over all vampires or finding a way to live in the open without fear. So, they give her this princess - like status. Also, it could be due to her relationship with Edward. If the Volturi see Edward as a threat or an asset, they might use Bella to manipulate him, and making her a princess is part of that strategy.