Why did Japan's overall exports fall during 1929-1931?

The effects of the Great Depression.

The main export of Japan was silk - a luxury product - and increasingly high tariffs worldwide and the credit squeeze, both caused by the Depression,made exporting almost impossible for all products, from all countries.

Japan lost its markets for its goods, like all other manufacturing nations. Trade barriers to restrict entry of foreign goods was one of the first remedies tried by all nations in an effort to slow the economic decline already worldwide in scope. By 1932 the total of world trade was one-half the level of trade in 1929. The US was one of the trade protectors, a policy that turned out to be the practice of "begger your neighbor" that made everyone poorer.

Money and credit were also worldwide problems; there was little finance available to support any trade, import or export, and not much production either, which also fell about 30percent or more in the same period.