As far as I can tell, simply because the war continued, though shenanigans are always possible. By 1942/43 the USAAF was using its medium bombers at low and medium altitude and had no need for a high-altitude medium. It was a good airplane, but not one the AAF felt it needed. Same goes for the B-33. If the B-28 had flown a year or a year and a half earlier it might have made it into production.