Answer: It is because of the close proximity with the Central Asian countries , Afganistan and Pakistan who combinely shares Islam as a common religion.
Islamic dynasties had very deep roots in areas around present day countries like Uzbekistan , kyrgyzstan , Turkmenistan , Tajikistan , Afganistan, and Pakistan. Moreover , Islam is the official religion , backed by the state , of these countries. As these countries share huge land border with China's western provinces , it had enabled to insert some influence over them. Additionally , the ethinicity also plays another dominant role.
As PRC , does not emphasise or promote any religion and due to high presence of followers of other community , the muslims are in minority.