Cheeky bastard, I spat out my coffee at "Deadbox". XD
As I understand it, it's largely down to lifestyle; the Japanese work very long hours, so a portable system is more convenient for them because they can play it on their commute as well as just during the very brief amount of time they have at home and not sleeping.
I think it also has a lot to do with the increasing domination of the home console market by Western developers, and Japanese companies finding it harder to compete in that market. A lot of them focused on the handheld market because doing so reduced development costs, and western developers generally didn't make big investments in that area.
Luckily the rest of the world stepped up. Japan's importance as a market for home consoles has severely diminished. Japan went from being responsible for 21.1% of PS1's to just 8.2% of PS4's, with the PS4 being the more successful product.
Same with Nintendo, 34.9% of SNES's were sold in Japan, only 12.6% of Wii's.
Once the PS4's sales finish, even without the Japanese region sales at all, it will still end up as the second best selling home console of all time. That's pretty crazy. How times have changed.
PS4 : Japan - 8.56m, ROTW - 18.48m.
That PS4 has sold over twice as many units in ROTW than Japan is also crazy...
Percentages by region of the lead console is kind of a bizarre way to analyze things. It leaves out a number of important factors. Most notably, you're ignoring the fact that the next closest rivals to the PS1 and SNES sold really poorly in Japan. The Sega Genesis and N64 were moderately popular outside Japan, so they reduced the SNES/PS1 market share in those areas, which increased the SNES/PS1 market share in Japan. The US portion of NES sales was 54% to about 44% for the SNES, but that didn't mean the US home console market was shrinking. Just that Genesis was taking up a big chunk of that market.
SNES + Genesis sales=20.6 million N64+PS1 sales= 27 million. Wii sales + PS3 sales= 23 million. So from the SNES to the Wii, the amount of consoles sold actually increased, but it was just split more evenly between the two main consoles whereas the SNES was essentially unopposed. There's really only a massive drop this gen if you look at the Swtich only as a handheld.