In 1486, Tsar Ivan IV (the Terrible) conquered the city of Kazan which had been under the rule of Moslem Tatars, and in remembrance of this, he decreed that from henceforth the Islamic crescent be placed at the bottom of the Crosses to signify the victory of the Cross (Christianity) over the Crescent (Islam).

Or maybe it's because when I was a teenager back in the '50s, my uncle Henry having denounced the Ku Klux Klan and been bombed for his trouble and had crosses burned in his yard, living under death threat took his wife and children to Massachusetts for safety and went back to South Carolina to face down the Klan alone.