The phrase spread around 2014 and 2015 as Black Lives Matter protests grew. Some people use it out of a sincere belief that naming one group is divisive. It is also used by opponents of the movement to dismiss its demands. Some politicians faced backlash in 2015 for saying it at campaign events.
The main criticism is that Black Lives Matter never claimed other lives do not matter. It pointed to a gap: Black Americans are killed by police at a higher rate than white Americans relative to their share of the population. Saying "all lives matter" in response shifts attention away from that gap. A common analogy is a firefighter spraying every house on a street when only one is burning.
A related slogan, Blue Lives Matter, emerged in late 2014 after two New York police officers were killed, and is used to express support for police. Understanding the debate helps explain why naming a specific group is not the same as excluding others.