Just to give an analogy using US geography, Xinjiang region would be like Oregon and Washington, and Qingdao (Tsingtao in the older spelling convention, a German colony before WW1) would be like Philadelphia. Except where Portlandians and Philadelphian both speak English, the Uighur do not speak Mandarin very well because Xinjiang has not been part of China very long (comparatively) and the Uighur have not needed to be very well integrated into Han Chinese society.