The issue is accountability. Police unions are much too strong, so the bad cops are reinstated even when they're on camera doing something heinous. How is the public supposed to feel about that?
This is why they're quitting, and this is why the drive to meaningfully reform police will be wasted this time around, too. It will be drowned in the consequences of the actions of radicals who push for simple solutions that are simply wrong, and all the nice middle-class folks from nice middle-class neighbourhoods who enable and support them, instead of calling them out on their bullshit.
This should be an unsurprising reaction to there generally being minimal consequences for individual bad cops, though.
The article doesn't say they're quitting because of slogans. From the article's citations, it sounds like they're quitting because they're essentially caught between a rock and a hard place; The public is angry with them because of little to no accountability and the police unions are too powerful and afraid to lose any bit of that power to be bothered to give an inch of accountability. It's an excruciating situation.