Indeed, portable consoles tend to get more use all around in Japan compared to in the west. One interesting factor in this divide is the rapid growth of the cell/smart phone market. While comparatively, the portable console market is stronger in Japan than in the US, the cell/smart phone market seems to have pretty grown evenly on both sides of the Pacific. Most of my friends stateside have voiced an opinion of "why get a handheld console, when I've got my phone?" whereas in Japan, most of my friends own both a phone and one of the major handheld consoles.
With smart phones basically becoming multi-purpose media tools (Anyone remember when all you used a phone for was to, you know, call people?), the casual and social gaming scene has pretty much exploded all over the place. Now, I'm not one to say which form of gaming is more legitimate, better, or more "hardcore" (I hate that word). I enjoy video games in all shapes and sizes and to say one type is better than the other is just closing one's self off from fun and entertainment. That being said, the effect of cell/smart phone gaming on the game market has been unmistakable. Nintendo and Sony have had no choice but to close the gap between phones and consoles by adding many similar features to their products (Cameras, gyros, GPS, touch screens, mp3 and video player functions, etc.) and as strong as the handheld market is in Japan, step on a busy train in Tokyo on any given day and you'll see a good percentage of the commuters with their noses buried in their cell phones, playing games, tweeting, browsing the web, watching videos, or watching porn (yes, I've actually seen people do this) as opposed to the few you will find using a Sony or Nintendo console. If anyone is hurting in the portable game market, it's not the phone companies.
Casual and social networking games have been an ongoing craze in Japan. Companies like Gree and Mobage have been pumping out games for iOS and Android phones and have been raking in the dough. The biggest advantage for cell/smart phone game developers is simply that people already have phones. Having game capability on phones is like spicing to a meal: You're going to eat it anyways, so you may as well add some flavor. The easy accessibility added with the inherent Skinner box-ish "collectible" nature of most of the games makes the cell/smart phone game market highly lucrative. (That, and the fact that once you sign up to play any of their games, it can be nigh impossible to deactivate your account.)
Sony and Nintendo, on the other hand, have been struggling to adapt. Nintendo obviously much less so. The 3DS had a slow start last year, but it has grown into a solid powerhouse for the company. And while sales numbers aren't as high as they'd like them to be in the West, Nintendo is doing well for itself in Japan, now consistently selling more 3DSes every week than any other console. The latest smash hit for the 3DS, Square Enix'sBravely Default: Flying Fairy has already sold nearly 150,000 copies and is currently sold out at most game stores.
As for Sony and the PlayStation Vita… Ah, the Vita. Don't get me wrong. I love the PS Vita. I own a PS Vita that I use and enjoy regularly. But personal preferences aside, there's no arguing with the numbers. The PS Vita currently sells fewer units a week than its predecessor, the PlayStation Portable. Compared with the 3DS, the Vita's performance over the past year has been "disappointing," to put it mildly. As a console, the Vita is a thing of beauty. But hardware specs and performance do not a successful console make. One noticeable move on Sony's part to try to stave their losses is the cross-console feature in several of their upcoming games.
This move essentially puts both the PSP and PS Vita into the same basket which allows for more copies of a game to be sold on all platforms, rather than tying sales to the performance on a single console. On the plus side, this will allow for developers to be able to make Vita compatible games with less fear of the game not selling well due to low console numbers, but on the minus side, this means that system-wise, games may be restricted to the capabilities of the weaker console. Also, even if they do manage to get a mega-hit game, the cross-console feature may result in fewer sale of the console they want to sell.
At present, it seems that the handheld console market in Japan has hit a stagnant period of sorts. Both Sony and Nintendo have their latest portable consoles out and seem to be warily dipping their toes in the water testing for sharks with less risky ideas, but too scared to try anything bold until they find a sure-sell solution (or a bigger boat). Meanwhile, the cell/smart phone market continues to lunge ahead without much fear, because whether the games sell or not, people will always need phones.