Part of it is because the witnesses know the DA will let the killer out on jail because of bail reform or Chinese flu or some other BS reason. Even in cases where a felon out on parole is busted with a gun on their hip, which should automatically send them to prison, especially in 'anti-gun' Chicago, the gang-bangers know they'll be out in a few hours, and the politicians will blame Kentucky for all the guns on the streets.

The cops can't promise to protect witnesses, because they know they can't.