The Chicago service has been suspended between January and March in the last few years largely because the 75w deployed in the shoulder season has performance issues.
Chicago gets pretty messy at times and the 75w operating at the limits means significant risk of diversion and disruption.
Lightly loaded this makes it unviable at the moment so they combine/route you over Phili or JFK or even heaven for fend Heathrow for those eight weeks.