I has to do with the fact that the US Dollar is the de facto reserve currency of world trade, Wall Street in New York is the largest financial hub in the world (and many of the other major financial centers are in US - friendly nations like the UK as well) and the relative sizes of the economies on each side. (politics.stackexchange.com)
Some say that bitcoin will become the new reserve currency of the world,... (bitcoinmagazine.com)
Since the dollar is the primary reserve currency of the world, the rating of the United States government has great significance in the global financial markets. (dailybitcoinnews.com)