Because academics around the world who are native speakers of non-English languages do much of their professional work in English, read English language journal

Because academics around the world who are native speakers of non-English languages do much of their professional work in English, read English language journals, and, if they contribute to Wikipedia, they are at least as likely to do so in English than their native language. As a result, English Wikipedia (for, say, mathematics) has vastly more attention and contributions, and is more comprehensive, than other languages.