Why is it so hard to maintain friendships as a female with other females?

For some women maintaining friendships with other women seem impossible. Often friendships "end" when a boyfriend comes into the picture, new friendships are gained [casting your relationship aside], and/or the oldest reason "jealousy". But why should a friendship dwindle because of a new boyfriend or new friends? I really don't understand. Men rarely have these issues. You would rarely hear a dude dissing his long time friend for a new girl or a new friend....