China isn't that socialist, there is no strong social safety net waiting to bail you out if you move to Tibet. If they can't make a life for themselves in Tibet, they would simply be left to die, so why take the chance?
Xinjiang, especially north, has a lot of farm land, a lot of natural resources that are easily accessible, as well a lot of urbanization and industry already. There is a reason it has 25 million people while Tibet only has 2 million. It is already 42% Han, which makes the ethnic tensions with the Uighurs a lot more explosive.
