Probably the same reason they allow the black racist Calypso Louie and his fruit-cakes of Islam to remain here in the USA.(No back-bone)
All those nut cases need to be deported regardless of race color or creed.
As I See It,
I work with a lot of immigrants who are not citizens, and there isn't one of them I wouldn't give citizenship to faster than to let someone like Duke back in here. Sometimes citizenship has the effect just like tenure on a campus, it just makes people a complacent malcontent, because we can't throw them out.