1. More passanegers arise because there supplies more seats. At the same time more seats are supplied because more passangers arise.
2. Tokyo centered governization;In the business Japanese make it habit to make face-to-face comunications, in-house, or with customer, or with the Beurau rather than to make it through telephone, fax, e-mail. The substantial negociation is made at night facing each other in a invitation party in a peculiar style.
This makes business demand in air trip very high.
3. Tokyo centered population; 60% of the Natinal population is concentrated in Tokyo and about 70-80% population of Tokyo residents have come from districts all over Japan in this 40 years. They go back to their homes intensively at summer and New Year, It's called homecoming rushes. This makes families seasonal demand very high(Please remember JAL B747 crash on 12 Aug 1985 with fatal of 520 people aboard. The summuer rush make things the worst.)
4. Airliners prepare aircrafts to fullfill almost the demand in the busiest season. (So in the most vacant season I have found only 10% seats are occupied at the cabin. In order to overcome less income in that season airliners have campain of selling 7000 JPY seats for any domestic destination(JAL) or 10000JPY (ANA) or of price cut in half around passanger's birthday weeks(JAS) or setting special prices for flights early in the morning and late at night.)