From the article - “The agency said Rapp and a fellow bicyclist were riding on a local road when Medina passed them in his car, stopped, and drove his vehicle in reverse. Medina then hit Rapp, and she was pronounced dead after emergency responders brought her to a hospital.”
Though we all think we know what happened, intentionally and/or knowingly are much harder to prove. It has to be “beyond reasonable doubt”. So if the available facts don’t back that up, a manslaughter case could be a much safer way to get a conviction. It could be bumped up to murder if new facts come to light as part of the investigation, though.