Shane Bieber hasn't been as good in this post season as Max has, that's it. Max has a better chance of going 4+, maybe 5, innings and allowing 3 or fewer runs than Bieber.
Also, this is an important game for us to win, because it's seen as the easier win than tomorrow (Ohtani starting pitcher) or Wednesday (Snell starting pitching I presume). So, we want to use out better starting pitcher to try to win tonight