Why isn't that police officer in jail ????

Same reason the police officer who choked Eric Garner to death in 2014 isn’t. He was fired, but someone will certainly hire him. And Barr waited until things quieted down to drop any federal charges. I’d love to be wrong, but I could see a similar outcome here.
Donald Trump even said “Police have to stop coddling these people “, so his remarks about how “sad George Floyd’s death is” were rather disingenuous.