It cost of airfare overseas.
In many parts of the Europe you can take the train to several different countries. Cost is the major factor. Time is a factor. Travel time.from Chicago to London is an 8 hour flight. By the time you add in getting to the airport, getting there two hours early, going through customs, then getting to your hotel it can be over 12-16 hours of traveling. That is two long full days of traveling. Not to mention the vast time change to adjust to.
There is plenty of different things to do in the USA. Two long coast lines, areas with mountains, deserts, large major cities, historic sites. We have the Redwood Forest, plenty of great National Parks, the Grand Canyon, Vegas, Hawaii, great skiing resort, Disney.