A likely answer might be: geography. The US is such a big country, with so many varied climates, and so many ethnic groups and neighborhoods, that many people simply don't feel the need to travel to Europe. Plus you can get very "foreign" experiences in Canada, Mexico and the Caribbean.
That's probably the reason that so few Americans are multilingual.

Europe's almost the exact opposite. Nations are smaller, closer together, share multiple borders. The next country is a short drive, train or ferry away. EU encourages cooperation. That's why so many Europeans are multilingual.