I think that the Devs tried to make every flyable plane in RoF as an availble one in the Career mode. Once the new plane will be relased, it will be added to the Career system, too. After the relase of 1.019, the Devs will take some break (summer vacations) and then, they will start adding new content into the Career (Aces, new paints, bug fixes)
The Bristol Fighter should be out very, very soon!
Lucas