There are clearly a huge number of people who may be sympathetic to the #BLM movement's ideals, but don't understand why #BLM rejects the phrase 'all lives matter'. In any exchange of ideas, it's important for both sides to understand how the other side receives what they're saying.
Some basic research into the apparent mutual exclusivity of 'Black lives matter' and 'all lives matter' reveals a lot of interesting information. I think anyone who doesn't understand why saying 'all lives matter' draws the reaction that it does from #BLM and its supporters, should check it out.
One common explanation why 'all lives matter' is received by #BLM and its supporters as racist, is based on the distinction between focus and exclusion. To #BLM, focusing on Black lives doesn't mean excluding all lives from mattering, or that Black lives matter more than, for example, white lives. This may have led to the other common phrase heard a lot lately, which is 'all lives can't matter until Black lives matter'. I interpret that to mean, from #BLM's perspective, 'first things first'.
Which leads to #BLM having a problem with how their message is received. They seem to place the onus to know what they mean, or want, on the person receiving the message. I think they need to clean that up if they want to get the attention and buy-in from enough people to make their movement stand a chance. Dismissing those who say 'all lives mater' as racists, is not the way to get there. Of course I could be way off base about that. Perhaps the SJWs here on the flip could provide some insight into #BLM's strategy and tactics.
I think #BLM also has a credibility problem with three issues. First, obviously, is the violence. Again, to get the attention of enough people to make their movement stand a chance, they need to get that under control.
The next issue could be summed up in the phrase 'Black lives can't matter until ALL Black lives matter'. To many, #BLM doesn't seem to care about Black on Black crime; for example the insanely high number of Black folks being murdered by Black folks in Chicago, and other large cities.
Last but not least is the fact that #BLM's leaders are admitted and avowed Marxists. Pretending that they're not, won't make that go away. The fact that #BLM wants to impose a system most Americans think would destroy what they've worked for their entire lives, is not the way to get enough people on board for their movement to stand a chance.