People who speak English as a second language are able to engage with a platform in which the majority of users speak English. People who only speak English or that and their local language are unable to engage with a platform where the language used is not their own or English.
More people are able to communicate with a shared language than with languages which are not mutually understood.
One other factor contributes: the U.S. has a large population which shares both a language and some culture. While multiple other countries may share languages, the populations which share a similar level of culture are smaller.
Then you have posts on social media being ranked in some way by engagement. One post may be relatable to 100k people, and five other posts are relatable to 20k each. The single post is ranked higher.