"all lives matter" was created to symbolise denying the existence/importance of police brutality and ethnic violence. By saying "all lives matter" you are essentially saying police brutality doesn't exist or that ethnic violence isn't present in America. Refusing to acknowledge the suffrage of disenfranchised people is (A) Ignorant (B) racist and (C) makes you an asshole.

Just imagine if there was a group in the government were unfairly targeting YOU and YOUR family, beating YOU and persecuting YOU. And then some rando who ISN'T being targeted turned to YOU and said "nah I don't think your suffering is real". You would be understandably outraged.