It seems obvious that a huge (and hard to solve) reason for this is simply language! There are many levels of language proficiency: Level 1: Can read and write it at your own pace Level 2: Can comfortably converse in a professional setting (little slang or cultural knowledge needed) Level 3: Can comfortably converse in a social setting (slang, faster speech, less clarification) Level 4: Can do all of the above passiv…
I'm not convinced that language is actually such an essential aspect. Cultural similarity seems like a much bigger one. For example, I've seen Indians with great English struggle a lot more to fit in and acclimate than French students with poor English but more shared cultural touchstones. As an American, I've found it to be much easier to make friends with Europeans than with students from non-Western cultures—even…
Edit: Another important factor (at least during collage) is the financial one. As a student from a poor country I had far less financial resources to afford a life style that American students enjoy. For example, I couldn't afford to go out or have a Car.