It actually made more sense for him to sign with an American League team, for the same reason. Using a DH in a game has always been optional (an option that, until Ohtani, teams basically took 100% of the time), so he could hit when he pitches in either league regardless.

But in the AL he could also DH when he wasn’t pitching, which he could not have done in the NL, he would have had to play the outfield on his off days.

He is by all accounts a pretty good outfielder but the general consensus was that playing outfield full time AND pitching every 5th day would have been too much of an injury risk.

So between the DH rule working in his favor, and the general preference that many Japanese players have to be on the West coast that I assume went into it too, he landed on the Angels.

As someone else said, both leagues have the DH now, but he had no way of knowing that at the time.

MLB also added an “Ohtani rule” a couple years ago that allows the starting pitcher to remain in the game as a DH once they have stopped pitching. It basically just allows a pitcher to DH for himself.

Previously once you chose to not use a DH, the DH was lost for the game, so Ohtani would have to stop hitting once he was no longer pitching. It’s a rule that basically only affects one guy and is pretty much only in place just to let everyone get what they want, which is to see that guy play more.