I'm guessing because it's a low-traffic route to a station that is a 68 minute drive from a major hub, and that is also serviced by 9x daily flights from that hub. Most pax probably get rerouted EWR-ORD-MKE or EWR-ORD-[rental/taxi] and are delayed no more than an hour or two. If you are short on crew or planes, that's a great flight to cut.