2 big reasons, first is that wobbles which we normally observe require that the star move enough to be detected on a shorter time scale. IE: if the orbit takes 100 years and we look twice in 5 years, the planet will have only moved 5% of an orbit and the wobble will be near 0. Second is the less mass and further the planet is away, the less noticeable the wobble. Something at 500 au is going to produce no measurable wobble in our lifetime.