I’d bet because it requires adults with better language skills.
We had a few exchange students when I was growing up and if the student was considered “mostly fluent” they’d come alone and have a group they’d meet with.
Students who were barely fluent often time had an adult with them.
I’d gather the exchange groups with less fluency require more adults, even if they’re not paid they have transport costs and whatnot.
That said, you’re totally right. It’s wild the article just goes “and of course it’s more money” as if it’s common knowledge why.