American Airlines is taking business class seats out of their Boeing 787-8 aircraft. Those planes are going from 28 business class seats down to 20, so that the “mini cabin” at the back of business class can be used for premium economy.
Chicago is American’s 787-8 city. The airline likes to base a specific aircraft type in each city to the extent possible, because it makes their operation simpler. When a plane goes mechanical, they have more of that aircraft. They have people used to working on those planes all the time. And they have the parts. They also have more crew qualified to operate the aircraft.