Uniforms

That's a pretty good question. After all, the Korean and Taiwanese leagues both use their respective writing systems for names on their jerseys. Various levels of amateur baseball in Japan (high school, college, industrial leagues) seem to be mostly Latin alphabet ("English letters") for their team names on their uniforms, but a good portion of them are in Japanese Kanji.

If I had to point to a likely reason for this phenomenon in Japan, I would say that it was due to a rejection of the Imperial charter that forbid the use of foreign names on the teams during "The War." I have no evidence to back that up, but it's a reasonable explanation of the observed history of team names.

I think it might be a little more complicated than that. The teams actually used the foreign names in English on their uniforms up until the middle of the 1940 season. It looks like (from looking through "The History Of Uniform," a book that shows all the uniforms worn by NPB teams up until 2005) in the middle of the 1940 season, all eight existing teams shed any English or Latin letters from their uniforms. I would assume that this is the time of the Imperial charter you've mentioned. After the war, all teams used English on their uniforms. I suspect that when teams started adding names to the back of the uniforms (the Whales were the first in 1964), they simply continued the tradition.

So the question would be - why did the pre-war teams use English on their uniforms in the first place? There's an explanation of sorts in the book "Taking In A Game: A History Of Baseball In Asia" by Joseph A. Reaves. Reaves describes the founding of the first professional baseball team in Japan as the "Dai Nippon Tokyo Yakyu Kurabu" by Matsutaro Shoriki. "Shoriki outfitted his team in natty uniforms emblazoned with kanji characters instead of Western numerals and ... sent them to the United States in 1935 to play a full season on the road." The team was fairly successful on the field taking on semi-pro and Pacific Coast League teams but "the only minor drawbacks to the team's popularity in the States were their kanji characters and their cumbersome Japanese bane. They rectified both by renaming themselves the Tokyo Giants and adopting a uniform identical to the New York Giants..."

So it would seem that the first professional team became the Giants and used English on their uniforms because they were barnstorming in the US for their first season. I would suspect that the other teams followed suit because the Giants were doing it.