US is part of the NANP which means they have their own system for beyond +1, which is shared with Canada and half the Caribbean, and so they were given the whole of +1 rather than +10, +11, +12 etc. all resolving to the same thing, or +10 being for about 10 different countries while +11 was for one

Then the Soviet Union wanted a single digit too, which is why Russia and Kazakhstan share +7