Perhaps it is the name given by English users when they first reached that region, but as we know after 1949, China adopted the pinyin and unified the spelling of English of names of place, Peking was changed into Beijing, something like Bombay of India into Mumbai, as the result of the nationalist efforts to make special identity. But Tibet is still in use, but not as Xizang. Why is it so? Hope it is not a political question.