Anecdotally from friends, I'd say the top three reasons are: * Lack of money * Lack of time off from work * Lack of English skills, making travel abroad intimidating. In previous generations this was counteracted by bus tours with Japanese guides, but those are seen as lame by the younger generations.
id probably add to that: Lots of hostile neighbors. Korea/japan (both koreas) relations aren't great, netiher are russia nor china. Meaning you gotta fly a long long way to get anywhere. Similar to how there are many people from american midwest whove never been abroad. living in europe all my life, almost everyone has been abroad here, but much much fewer have ever left europe, which is essentially what japanese peo…
There is also internal Japanese tourism - such as to Okinawa or Kyoto