Explained answer : Well, you have to understand that the British were taking a lot of resources from China from the 1800's on up until their mandate in China ended. Start with the Boxer rebellion and then work your way up to the early cold war when England and other nations sided with Chian Kai Shek (2sp.) There was a lot of bitterness over the way the British administered many of their colonies from South Africa to India to Jamaica to China. Nationalism is sort of a byproduct of that. Look into the Boxer Rebellion, though.
Easy answer : because they adopted a policy known an imperlism.