Because English Wikipedia is by far the version with the most readers, and therefore the one where most time has been invested in making an article good.

In my experience, non-English Wikipedia articles are great for "insider knowledge" about topics and/or local figures who are deemed not relevant enough to the English Wikipedia editors. But outside these niches, their texts tend to be both shorter and less polished.

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.

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

Source?

I don't know about Maths and other STEM pages, but modern history and politics (last 100-150 years) pages in many non-English languages are highly questionable or even downright lying.