The problem with mathematical realism (which, btw, see also), is that it's challenging to justify the simplicity of our initial state - Occam is not a fundamental law of physics, and almost all possible universe-generating laws are unfathomably large. You can sort of justify that by saying "even universes with complicated initial states will tend to simulate simple universes first", but that just leaves you asking why the number of simulations should matter at all. (I don't have a good answer to that; if you find one, I'd love if you could tell me)