And a lot of the ingrained prejudices in the US are specifically black - not native American or any other race.
The reason black protesters have insisted on #blacklivesmatter is to keep us aware that there are special problems that black people face that they want on the table, at this minute, while it's hot and there's a chance to get something done.
#Alllivesmatter is true, but it would just swamp the issue. This is the same problem feminists have when people start proclaiming themselves #humanists and assuming hipster-savvy poses about it. Of course we are all aspiring to a world of gender equality, but right now women have special problems that we still need to call particular attention to, such as genital mutilation, honor killings, lack of resources for rape treatment in the military, etc. Being a Feminist means you have decided that some issues, categorically, need priority attention - and by adding the -ist to your own identity you dedicate yourself to that cause.
Bringing justice and equality to native americans is just as valid as all of the above. They are part of the broader Civil Rights and Social Justice movements - but we all have to admit that nothing much gets done under those diffuse topics. So the native americans must be native americanists, and when the time is right, they should cry #nativeamericanlivesmatter!