You may want to rephrase your question and include a time period. If you mean during the modern period, Japan did "attack" the rest of China - during their height, Japanese puppet states in China (e.g. Manchukuo and the Reorganised National Government of China, a.k.a the Wang Jingwei regime) and areas under direct Japanese control occupied much of the heavily populated areas of China during the Second World War. Japanese rule in China extended from Guangdong (Canton) to Beijing, and along most of the Chinese coast. The Chinese government under Chiang Kaishek was forced to relocate to Chongqing, the furthest inland any Chinese government has based their capital.

Rana Mitter's "China's War with Japan, 1937-1945" is perhaps the foremost work on the Second Sino-Japanese War.

If you are referring to the First Sino-Japanese war, Japan's interest was in securing the Korean peninsula. Thus, much of Japan's strategy was focussed on Liaoning and Zhili provinces.