Because they think the cops are the good guys and that civilian review boards exist to hinder them and prevent them from keeping themselves and everyone else safe. Especially because they think that the people who’d join a civilian review board are anti cop.
On the left we’re aware of the police problem but there’s a lot of pro cop and anti criminal propaganda in American culture still and many people still believe whatever the cops say.
If it's anything like where I'm at, cops (and troops) are heroes and therefore any criticism (or even showing the slightest doubt) of their actions is seen as anti-American
They believe that civilian boards and panels are made up of "woke" activists that want to screech at them and defund them. Since these panels aren't as hardline conservative/fascist as they are, they're "libtards" who shouldn't have a say in these "conservative" matters.