Every country has regions that are more or less developed than others. An example from the United States is California, which has a highly diversified and vibrant economy, or at least it did until the recent economic downturn. By contrast, the state of Alabama is more rural and has less diversified economy thereby making for a lower per capita GDP. China is the same, with eastern provinces having diversified and well established manufacturing and service economies, while western provinces are still developing.

because they don't want to work had and make a developed region…
other reasons are climate and location…

far from ocean, no transportation means by shipping.

xinjiang mostly dessert, tibet mostly cold mountains—–not fit for human to live at.

even in ancient china, these 2 regions not very populated.