Because Warren availed herself of programs designed to assist ACTUAL native Americans...which she is not.
That, plus it's always fun to make fun of Left Buffoons...like Warren:
Cherokee Woman Blasts Elizabeth Warren: 'We've Asked Her to Stop' Claiming Our Ancestry
http://insider.foxnews.com/2018/01/2...rican-ancestry
..and that's not even counting all the money she took from Wall St. firms while "denouncing" Wall St. firms; that's an entirely different area of Ridiculing Warren...which is becoming a sport amongst the well-informed.