Because Chicago Dem Michael Madigan has been the Speaker of the Illinois House since 1983. He uses all the tools he can find to reward those he likes, and punish those he does not like. If you’re a Dem in the IL House, you probably do not want to be on Madigan’s bad side. Also, his daughter, Lisa Madigan is the IL Attorney General. That means that she can choose to pursue/not pursue possible criminality in IL government. Madigan also controls the drawing of state legislative districts, and also controls which people are elected to judgeships and the legislature.
Surprised that no one has mentioned this.