So explaining why something didn't happen is a tough proposition, but I would suggest that a bit part of this is the premise, as baseball didn't become popular in Japan post-WWII, rather it was already a very popular sport by that time, and the roots of Japanese baseball date to the 19th century. While baseball was used as a point of commonality during the post-war environment, it wasn't a post-war import. This older answer goes into more detail on Japanese baseball.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!