Because they use slave labour to manufacture and trade products at a cheaper price than any western country can afford. They have 13% of their population (approximately 130 million) living under the poverty line (less than $US1.25 a day per person). This is only possible because of the strict political and military mono party domination of the communists. No Unions and no protesters are able to break this hegemony. When they tried years ago thousands died (Tiananmen Square protests).
Wages are on the Rise in China,and its putting pressure on the Chinese Economy,long term some companies who have moved into China are now questioning wether their advantage will be maintained.