I taught ESL for 30 years, and I know why. Many studies have shown that just two years of intensive English brings the non-English speaker up to near native speaker level.
However, that makes the students lag behind in other areas. The social costs of having older students in other classes was deemed too great, so they went with bilingual classes in which neither the coursework nor the English was mastered.
Go figger.
A few years ago, my wife’s cousin and his family came here from Cuba.
Their two young children, didn’t speak a word of English.
Fast forward a few years, and those kids speak English....PERFECTLY!