It is not that they refuse to assimilate, it is that they can’t.
Muslims, irrespective of the country of their origin, (except Iran or Turkey) were raised in such way that they can never assimilate into Western culture.
True Muslims unanimously abide by and live under the laws of Sharia and not the Constitution of the United States of America or other man-made laws in other countries.
People who live in the West must understand that to be a loyal and faithful Muslim, one must adhere to and perform many rituals, as specified in the Quran by Allah and the Hadiths/Sunna, every waking moment of his entire life.
Disobeying these rituals does not make one a moderate Muslim, but rather it would make him a non-Muslim, facing an uncertain future. It is not their fault that they adhere to their customs and traditions.
They were born with it. It is our fault that we bring them here and force them to assimilate.