In just guessing, but I bet its connected to the fact that you are measuring American English against the other languages. Its tightly connected to the fact that the dollar is the de-facto monetary standard to trade in the world. Other countries have to speak English to get involved but since we already do, there isnt a real reason besides intellectual curiosity to do the reverse.So we are uni-lingual while other countries have to be able to buy sugar in their native tongue and speak English when they want it shipped to their country. The truth of the matter is that virtually every modern immigrant -leaving out the Europization of America- is multi-lingual. If it were the basis of intelligence Americans would be behind the scholarship. So not an mysterious situation. Its because trade starts in many ways with the dollar, so no need to be multilingual.