It goes out of its way to be exclusive when it could be much more effective if it was inclusive. This is especially important if you are trying to draw empathy from a group you feel hasn't shown it due to an us vs them sort of divide (which is what racism is). If you really want to make a difference you need to make people feel like we are all part of the same group, and underscore our equality within it, instead of leaving the impression that we are a bunch of separate tribes in competition. It helps to show that you are out for fairness to all, and that you are not just pushing your particular subgroup. Do you care about black rights or human rights? Black lives or all lives? Further note that #blacklivesmatter doesn't just exclude white people. It also excludes latinos, asians, and everybody else who isn't black or white.
#blacklivesmattertoo would avoid that. 3 extra letters. #alllivesmatter would be even better. Because all lives do matter, and if some are being treated differently than others because of their race, gender, ethnicity, sexual orientation, or whatever, that should be addressed for what it is. This isn't just a black people issue. This is an all people issue. We have all been treated unfairly at one time or another to some extent due to some arbitrary trait beyond our control. Some far more than others, yes, but we should all be able to relate, have empathy, and therefore care, if you make this inclusive instead of divisive. The holocaust wasn't just about jews, and all lives matter because they are lives, not because they are black or white.