The EPR, EPN, EPP, and EPY films were Kodak's first E-6 films. They were "technically" bettered by the T-grain E-series Ektachromes, which have much finer grain. But the color saturation levels of the original films are lower, and thus preferred by some users. If you're shooting 4x5 (which was much of the market for those emulsions), the grain isn't really very important. EPP has somewhat enhanced saturation, very nice for landscape photography. EPN was particularly unique in having very limited spectral response into the infrared color region, so it was much less subject to anomalous color response. This made EPN particularly popular in catalog photography, as fabrics with lots of reflectivity in the IR region came out the right color. Also, many folks were "used to" using these older films for CMYK separations and printing, and didn't want to adapt to the newer films. So Kodak kept these early films in production for a long time -- but at pretty steep prices compared to the E-series Ektachromes.
Digital has taken over most of the remaining niche market for EPR, EPN, EPP, and EPY, so the films got discontinued. The few art photographers using EPP just get the shaft, there's no market to subsidize their habit...
As for Japan, perhaps the remaining type sold there was popular for photographing cherry blossoms -- which is a "big thing" in spring in Japan. It probably was just the remains of the last master roll, not like they made any more of this after announcing discontinuance.