I'm curious to know what stupid, easy western games people are talking about. I'm sure I'll hear a chorus of, "SKYRIM!"; that game is plenty easy, but the point of that game is something other than mastery of mechanics. It's an exploration game more than anything. And I liked it just fine. On the other hand, we have titles like The Witcher 2, S.T.A.L.K.E.R., FarCry 2, Metro 2033, The Void, GTA, and Starcraft that can be extremely challenging single player western-developed games, and I loved those too. You, and Chilimac the Lamer Namer, should try them.
I'd argue that games are easier in this day and age because A) we're a lot farther from gaming's arcade roots than we were in the 16-bit days, and the point of gaming has evolved past its high score chasing, quarter stealing roots, and B) we're better at making games now. No one could have made Super Meat Boy in the 16-bit era (an exceptionally difficult western game, if anyone is keeping score), because the level design in that game is astounding compared to most 16-bit 2D platformers. It's intricate in ways that only Nintendo was doing back in the nineties.Games are much easier than they were back in the 16-bit days, no doubt about it. From a personal perspective, I do miss that sense of achievement when finishing a game, the feeling of overcoming a challenge and honing my skills to reach a deeper level. A lot of modern games don't have that depth to the gameplay, but that's because the industry has changed and gaming is a very commercial form of entertainment these days. They have to pander to the mainstream to survive.
While we're at it, despite their intricacy and craft, the Mario games from the 8 and 16-bit era are pretty easy. I beat Mario 3 and Mario World on the very first day I owned them. Those are not difficult games. On the other hand, Lost Levels took me a month of being very angry to beat, and it's universally reviled (and rightly so).
I think it's insane to quantify the quality of a video game based on it's difficulty. I love Cave shooters, and those are brutally difficult, but I also love the Mario games, Little Big Planet, Skyrim, Deus Ex, every horror game ever made, etc. etc. etc. If you enjoy a hard game, great! That doesn't mean it's better than any given easy one. And if you want a challenge, there are games out there that will give you one: plenty of them in fact. As a bonus, they're often very pretty to look at, too. Witcher 2, I'm looking at YOU.
And now: the Original Point!
From what I've read and heard, Japanese gamers like some western games just fine, but the Vita launch line up was pretty much trash. Uncharted is a 'meh' entry in the series, Ridge Racer Vita is an insulting nightmare, Ultimate Marvel 3 is considered kusoge in Japan and very few players like it or take it seriously, Little Deviants is trash all the way, the golf game did pretty well, but it's yet another hot shots golf game, BlazBlue is a port of a game that most of the people who play it already own on consoles... The machine itself is expensive, you need an additional (also expensive!) memory unit to play some games, and Japanese owners have a huge library of PSP games they can download, too.
Consider also that the 3DS now has Monster Hunter and probably Dragon Quest at some point, whereas the Vita does not. Do you see Monster Hunter or Dragon Quest on there? Nope! That's probably another pair of reasons why it sold badly.