Full Idea
Rawls rejected the cosmopolitan extension of his theory because he thought it failed to respect the political autonomy of 'peoples', which was his term of art for societies or political communities.
Gist of Idea
Rawls rejected cosmopolitanism because it doesn't respect the autonomy of 'peoples'
Source
report of John Rawls (The Law of Peoples [1999], p.115-8) by Andrew Shorten - Contemporary Political Theory 09
Book Reference
Shorten,Andrew: 'Contemporary Political Theory' [Palgrave 2016], p.267
A Reaction
Interesting that you might well start with the concept of 'a people', prior to some sort of social contract, but end up with rather alarming conflicts or indifference between rival peoples. Why should my people help in the famine next door?