It's about how they die, and by whom. A black man was murdered by a police officer who is supposed to serve and protect him.
There are many cases of police officers killing black people due to racial profiling, of course. But I've seen a few videos and it always seemed to me the police officer was panicking or acting stressfully in the situation (like the infamous video last year when a police officer shot a black man inside his car after freaking when the black driver calmly told him he had a gun as he slowly went for his license). It was horrible, but you could see the police officer freaked out. It was a horrible mistake caused by inexperience and racial profiling.
WARNING: Video contains disturbing images - DO NOT WATCH IF YOU'RE UNSURE YOU CAN HANDLE IT
The George Floyd case is different, however. The coip murdered him in a calm, orderly, calculated fashion which almost seemed surreal. It was like the Law was telling black people "We can end you whenever we feel like it, and you just have to sit down and take it." This is what made thousands of black and white sympathizers to go apeshit and protest (and riot). It's a slap in the face of human rights for many people and they want change and justice. You can just equate this to a murder charge or a terrorist hit. These things happen by people who want to hurt you.
George Floyd was murdered by a man who was supposed to keep him safe, and was murdered in a way that rivals dogs being put down.