Because they cover their bodies even in the summer and are severely deficient in Vitamin D, a great number of Muslim children are born autistic and suffer from all kinds of syndromes.
That's what you get Britain for bombing German civilians in ww2 for democracah'.
Now you will be Sharia land.
Most people are Vitamin D deficient. Especially dark skinned people living in Northern Europe need more exposure to the sun than white people to stay healthy. Food cannot compensate for the lack of sun exposure and even vitamin supplements aren't adequate unless they are of good quality and in the right amount. Most Muslims come from third world countries and aren't educated enough to know they must supplement themselves with vitamin D.
Britons believe in freedom of religion, unlike you Southern Europeans. If they want to wear a hijab, let them. Would you rather they become Christians and intermarry with Europeans?