I suspect that Cook County is a special project. You can’t have half the county in prison, especially the Chicago Department of Streets and Sanitation. Who would pick up the garbage? At some point the Chicagoans will need to clean up their own mess.
Multiple decades ago, when I was young and naive’ and fresh arrived in Chicago, the son of the precinct committeeman offered to see if they could get me a job in Chicago city government. Part of the deal was to go out and get out the vote and keep track of the voters. He took me to see George Dunn, but Mr. Dunn was not at his office that day and I was not installed as part of the Chicago Democratic Machine. Not quite illegal, but not quite not illegal. It was from a time before I was politically aware.
It could be that they don’t want to try to have those cases tried in Chicago, so made sure not to do indictments there if at all possible. Plus, if Obama can get a sealed divorce released, he can sure get sealed indictments leaked or given to him for a heads up.