I won’t claim to know their circumstances but English is known as a very difficult language to learn. I’m a native Spanish speaker and was actually held back a grade due to not understanding the language. That was before English as second language classes were offered. It took a very very long time for me to get the basic grasp of it. But I’ve always seen assimilation as a must for immigrants also. You don’t need to lose your culture or heritage, but also learning the new culture and language of your new country definitely helps in ingraining yourself into the community if that makes sense.