Socio economic reasons are a big part of this. Providing education for ESL students isn't cheap. Most ESL is for low income students whose parents don't pay for private lessons.

If you take all middle class and above students and above the US figures well compared to other countries.

So it's not that how we go a out educating is off... It's just that educating such a diverse student population with the economic and social characteristics is more difficult than when you have a more homogeneous culture.