It's also worth pointing out why the use of Navajo code talkers was so successful. Part of that is that it's from a language family that was poorly known by linguists at the time. But even within that language family, Navajo is pretty different (compare how different English is from other Germanic languages, or indeed most Indo-European languages). It's also a fiendishly complex language; like, there's no regular verbs--that would make attempts to crack it via standard cryptanalytic techniques rather more difficult. (Also worth pointing out, it was a code within Navajo so that even when the Japanese pressed a Navajo POW into translating the messages, they couldn't crack it).
More generally though, Japan now and Imperial Japan in particular was very big on there being "one Japan" with one Japanese people and one standard language. Dialects were not encouraged and Okinawan, which is really a different language, was ruthlessly suppressed (kids punished for speaking it in school etc).
Those letters mean: "Welsh not". Back in the day, Welsh children would be punished for speaking Welsh (instead of English) and made to wear a wooden board with WN on it around their neck. The board would be passed on through the day to the final transgressor who would receive a thrashing or similar punishment. Generally all of them would get a thrashing anyway - it was good for them!
Edit: Not, not no ie WN: "Welsh not". Small but important difference.
Japan had one and only one national language. Japan didn't have the culture of thinking of itself as a melting pot where many languages might meet. It probably wouldn't have thought to ask that question at all "what's the most diverse language that the Japanese speak?" In practice, we can assume that's also why it confused the Japanese for so long in the war, it wasn't a question to think of themselves, it probably was a hard question to ask of Americans if they saw "English" as the national language they might also not think to ask "what other languages do Americans speak?" or maybe even "why isn't this English or a derivative of/code for English?"
Uniqueness of Navajo and other Native American languages was that they were all complete multi millennia isolate from English thanks to how USA came to be. Japan wasn't like that so dialects aren't divergent enough for this use case. Ainu might be a candidate but it's way too high profile.