Here are two articles that should help with your questions one is a few years old but the information in the article addresses a lot regarding why Japan's birth rate is so low and still falling.
Italy is also facing the same problem. Birthrates in multiple western countries have actually started to decline there are many reasons behind this. But we will stick to Japan for now.
BBC NEWS | Asia-Pacific | Gender issues key to low birth rate
A special report on Japan: The dearth of births | The Economist