Seriously? What's with this question always coming up? I mean it's kind of obvious.
Why does PS4 exist in Thailand? New Zealand? Nigeria? South Africa? Ukraine? India?
The answer is the exact same.
It's a bit different in all of those countries, they're interested but they're just small and you get what is expected.
In japan not only are they not interested but it's a huge territory, lots of retailers to organise distribution for a decent sized country.
Selling 1 console in a sample of 100 people is better than selling 1 console in a sample of 10,000 people.