AirTran and Southwest are ferocious competitors. So people have voted with their wallets away from Frontier. The result: Frontier is putting its planes where they can actually make some money where the market is big enough (in MKE it apparently isn't) for competition on most routes they serve, where demand is high enough that they can fly either high-capacity planes or charge higher fares, or where they're the only game in town (so they can fill planes with middling fares). Pick any one, two or three of the above, and factor in Frontier's focus as a low cost carrier (ultra-low-cost to get technical, but Southwest will compete with ULCCs for the heck of it if it allows them to reinstate a monopoly later). The result: MKE isn't a good fit for them as a hub anymore. The city will have Southwest in town, United and American down the road in O'Hare, Southwest further down the road in Midway...and hub or leisure service from the likes of Frontier, Delta, etc. (actually Delta might pick up more routes if the business demand is there).

Given the choice between this and continuing to bleed (potentially compromising the Denver operation, which itself is in a very competitive atmosphere but serving a much larger market) Frontier chose to pull out of MKE, except for a handful of leisure flights and service to DEN. If an opportunity exists for another airline to serve MKE profitably (without getting hamstrung by Southwest or Delta cutting prices on a given route), maybe another airline will step in. But if people want something like the old Midwest, they don't want the new Frontier anyway...not with ever-larger planes, a la carte options for everything and flight times that might be only a handful per ccity pair per week...

Full disclosure: I've enjoyed Frontier's Denver presence for the past five years, flying them marginally more than Southwest despite parents having a Southwest credit card. I wouldn't wish Frontier leaving an area on anyone, least of all Coloradans who get great air service from their capitol thanks to two or three airlines competing on a given route with Denver as a hub. That sort of competition doesn't happen in cities as "small" as Denver very often.