Because in Fallout till New Vegas. There has been almost no contact with any other country with exception with Canada. Also try comparing the contacts the countries are making now compared to Fallout. Its obvious they have been screwed too.
I'm not saying they arent messed up - but there is no mention of them at all, positive or negative.
How do you know there is no (or almost no) contact? Answer, you don't.
Noone mentioned it sure, but they games only play for a short period, if a bunch of travellers with funny accents show up for a few days and leave, they arent going to keep talking about weeks/months/years afterwards. Just a bunch of wierd travelers, hey look, heres another weird traveller in one of those old time vault suits.
I am not saying that there is contact, but Absence of proof is not proof of absence!