Because they mismanaged their "Lost Decade". When Japan's real estate bubble collapsed and they went into recession they never entirely got back out. So they had a level of spending, but their tax receipts never returned to pre-recession levels to pay for it. Then in their stimulus spending, they dribbled it out for useless projects instead of investment spending, and never did enough at one time. In the meantime, their demographics kept getting worse, making it harder and harder to return to the "economic miracle" days. So it's really some 20 years of just letting problems drag on and on rather than dealing with them.