I agree that MDW is easier to use to reach downtown.

However - In the summer time when Frontier wants to fly ILG-MDW, hotel rooms in downtown Chicago are quite expensive or sold out, easily twice what it is between January-March. Even the MDW airport hotels are fewer in number than ORD airport hotels, so those rooms become expensive. There are about 8 decent hotels by MDW (Marriott, Hilton, etc.) but I think over twice or three times more by ORD including more 2 star and 3 star hotels, which likely gives the leisure pax more choices.

A quick look at Starwood's site, and this is for Sep. 23 (technically after summer), and the downtown rooms are all over $250/night. They have about 3 ORD hotels to choose from with rooms in the low $100s, but no MDW hotels.

So, for the pure leisure crowd like Frontier's run on MDT-MDW and ILG-MDW assuming this is summer, some customers might be priced out of the downtown hotel rooms or unable to attain them, and if they want to avoid a rental car, the only other option is an airport hotel.

I've wondered if MDT-MDW would have performed better on MDT-ORD with flight time mid-day or later so that pax coming from 2 hours drive away (from MDT) can use it, but anyways MDT-Chicago is likely over with Frontier. Atleast ILG is fortunate to be on the seasonal list.