Money.
Theaters buy movies that are likely bring in money, in this case if Fox made new X-Men movie, every theater in the f*king world will show it because:
1) Fox has got hundreds of millions to put in to marketing, no other country could has got so many big production companies that have so much international appeal. What would be the point for US cinemas to show small french movies? They don't have the money for marketing, if no one knows it exists then no one is going to see it, quality doesn't matter. Transformers - a piece of sh*t movie that has hundreds of millions to push in to marketing to inform every single person on the face of this planet that they just crap a new one out.
2) It's a trusted brand, like Coca Cola, they're movies have made a lot of money in the past, if cinema buys X-Men and than it brings in thousands of people who want to see action, then they sure as hell will buy every movie Fox is generous enough to sell them.
3) Hollywood has the biggest industry, most talent, people that are willing to invest in movies, TV industry, award shows, there already exists a whole system for producing, and marketing movies, it attracts people from all over the world, no other country can say that, not in this scale.
Other countries can't break in to that market because, there needs to be something to start it all. If you have never made a big budget movie and are seeking funds to make one, no one is going to give you that money, there is no evidence that you can make or sell anything worth while. There are not enough educated people that work in the industry, in other countries movie industries are small and self contained, selling movies mostly to neighboring countries no international appeal.
Hollywood created star status, celebrities, turning people in to untouchable superstars. They are marketing actors, not just movies to international audiences. How many famous foreign actors you know and would go to see a movie for if you don't live in US? How many US actors do you know and would go see a movie for? Mostly it's - US actors, maybe some for neighboring countries and your own and that's it.