Southwest runs a pretty small operation at IAD. I believe only 3 gates and about 8 daily flights (On weekdays, 3x to ATL. 1x to FLL, 1x to MCO, and 3x to DEN - and less service on weekends).

To my knowledge, this flight didn't exist in the past.(edit: as per others' comments below, these flights did exist pre-DCA service) The WN schedule is set through Aug 6 2018, so even if WN introduced that flight, it wouldn't be until after that. My guess is that it's unlikely though, given the large WN presence at BWI and at DCA. My guess is also that the demand for IAD-Chicago traffic is filled by United's flights to ORD and given the limited number of slots, WN doesn't think it'd be profitable. It does look like there's a lot of capacity across different airlines between the DC/Chicago area airports - just not between MDW-IAD. Unfortunately there are limits to the numbers of gates and planes WN has access to. (A similar thing happens out here in California, for example - flying out of OAK you can go to any Southern California airport on WN, but out of SFO the Southern California airport options are a little more limited).