Only the sales of the original trilogy were better in the west. Ever since AA4, Japan has been outselling the games in NA and EU combined. That's also why they went with a digital only release for AA5 in the west, and didn't localize AAI2 or Dai Gyakuten Saiban, because they'd likely spend money they couldn't earn back with sales. It's no secret that Capcom hasn't been doing well in recent years, so they want to save money where they can.
And I don't see why a game made in Japan shouldn't be released in Japan first. It's the market these people develop their game in, so also the market they should invest in. The only exceptions are when games get a (near-)simultaneous worldwide release.
The game also deals with Japanese culture a lot, and due to the decision of the localizaton team that worked on the first game to change the setting from Tokyo to Los Angeles, there's a lot that has to be done in order to prepare the games for a western release.
Anyway, I'm gonna get the game when it's out. AA5 was kind of a disappointing installment in the series due to how easy it was (and how many hints it gave), but the writers did say they'd add an option to turn the hints on or off. Whether that has any effect on the difficulty of cases remains to be seen, but I'll give them the benefit of the doubt.