It's mainly just speculation based on my limited experience training with Japanese club runners, but you're welcome.
I'm an American who lives in Japan and has followed Japanese T&F for more than 30 years (I covered the 1991 Worlds for the Japan Times -- saw Mike Powell's LR WR and Carl Lewis' 100m WR!).
My view that Japanese neglect speed-endurance training is also based in part of what Suguru Osako did while with the Oregon Project, where there was presumably a greater emphasis on such training.
Even though he's more of a marathoner, he still holds the NRs for 3,000 and 5,000 and ran 3:40 for 1,500m, which would put him among Japan's top 1,500 guys this year, at least so far (and I don't think Osako even ran many 1500m races).