No. 1: Ohtani won't be the ace elsewhere
If Ohtani decides to go with Texas or Los Angeles, chances are he won't be the ace at either spot.
The Dodgers have Clayton Kershaw's contract to wrangle with this off-season, but let's be real, it's just a matter of meeting at the right number for him and he'll be a Dodger for life. If he goes somewhere else, it might be towards the end of his career, but he doesn't appear to be slowing down anytime soon. Not to mention their secondary starters of Walker Buehler, and (when he comes back from injury) Dustin May.
If he were to go to the Rangers, they have not one, not two, but three already established starters who would be an ace on any other squad in Max Scherzer, Jacob deGrom (when he returns from injury), and Nathan Eovaldi.
Sure, he won't be pitching in his first year of any deal, but once he is he will be looking to lead the staff and make sure they make it into the playoffs under HIS watch, not someone else's. Plus with the money he is commanding of almost $100MM per year, give or take $10MM, any team in their right mind is not going to pay him that to be their number-two, number-three starter or (in Texas' case) number-four starter.
The Red Sox have the best set up for him where he can walk through the door, and immediately become their ace. He won't be day one seeing as he won't be pitching in 2024 (as alluded to in the previous paragraph), but just to know that he WILL BE leading the rotation as early as 2025, should be enough for him to heavily consider Boston.