KORD is only about 15 miles away, they could of landed there and bussed the people to KMDW. SW is point-point so the paxs didn't have to make any connections
MDW
is a connection/transfer point in Southwest's system. Not a hub like DAL or PHX but busy enough.
As for ORD, I doubt there would have been slots and certainly no gates. A 'local' divert to IND or STL (WN stations) would have been more likely, but if all area WX was cr*p maybe that wasn't a workable option.