A game is aimed at an audience and not all audiences have the same means of purchase. That's why there are variations the digital tie ratio. RPGs tend to do poor digitally because they offer little after the completion which means you get people who want to sell the game right after competing it. RPGs can also have very dedicated fans, who want to get the special edition items. Neither of these things are possible with digital versions.
RPGs have short legs because once you finish it it has little value to hold on to. Monster Hunter built its entire appeal on social multiplayer. One of these titles is a way safer digital purchase.
Well that makes sense too. Still even with RPGs I would say that digital option saves at least some sales if there retail shortages.
But Doom is one of the best shooters in years and it bombed in Japan. More like Japanese don't buy single player focused shooters. No matter the quality.