fairviewroad

I'd guess that's because the MDW station is used far more often by local community residents than the ORD station. The MDW Orange Line station is very conveniently located to some residential neighborhoods and is also a connection point for local city bus service. Many of the passengers at the MDW Orange Line station have nothing to do with the airport itself.

So a surcharge there would penalize local folks in a way the ORD surcharge would not. Perhaps some local alderman/community leaders spoke up on their behalf, or perhaps the CTA wisely decided not to even pick that fight to begin with.

Whereas nobody would ever use the ORD station if you weren't specifically using the airport, so it doesn't really have the same impact on locals.

Another note: The surcharge is an extra $2.75 for leaving O'Hare, but is not levied on people arriving at O'Hare from downtown. So if you fly to Chicago, and use the Blue Line both ways, you're really only facing an extra $1.38 each way. That's really not too bad, though it would be nice if they (finally) got around to offering express service.