Easy. America is so big, that it is almost in a world of it's own. You have enough quantity of sports inside the country, that there is no need to look at the international sports. When ESPN, Fox Sports, or newspapers for decades have been covering 99% only American leagues and stars, the people have adjusted to only care, or really know, only them.
Of course, the capitalistic, money grabbing (and absurd in my opinion) system of having as many matches, as physically possible, prevents any free time to even attempt watching anything else. 100 matches for your basketball and hockey teams, 200 matches for the baseball one, 40+ Nascar races, golf every week... Good luck trying to get someone to start watching ski-jumping, cycling etc.