People accept circumcision because a centuries old patriarchal, woman hating, book told men to chop off their foreskins to show that they had a special bond with God (women did not have to perform a mutilation to show their devotion to God as God did not want a special bond with women - they had periods and ate apples given to them by snakes and other such yucky things).
This book became religion. people used to die and kill for religion and everybody is a bit afraid of going up against it because they will be called a bigot, or accused of interfering with someone's beliefs. There are lots of problematic aspects of religions that do need tackling, circumcision is one of them. Not necessarily feminism's first job or most pressing issue though. Although I am sure many feminists do consider it a pressing issue and are tackling it, we're not a hive mind you see and one of the good things about that is that we can wage war on lots of different fronts according to our own priorities.
(FGM is cultural rather than religious, both Christians and Muslims are known to perform it in parts of Africa - Somalia I think is the main place.)