Why isn't runway 4L/22R at KMDW available as an emergency runway? Chicago-Midway (MDW/KMDW) has five runways, oriented along the diagonals of the square airport.  The longest two (13C/31C [1988 x 46 m] and 4R/22L [1964 x 46 m]) are ILS-equipped, and serve as the airport's primary air carrier runways; the other three (4L/22R, 13L/31R, and 13R/31L) are used primarily for small general-aviation operations.  Of the three, 13L/31R and 13R/31L are available for use by larger aircraft in case of emergency:


  Chicago Midway International Airport covers just over one square mile (650 acres or 260 hectares) [...] and has five runways: [...]
  
  
  13C/31C: 6,522 ft × 150 ft (1,988 m × 46 m), air carrier runway, ILS-equipped.
  4R/22L: 6,445 ft × 150 ft (1,964 m × 46 m), air carrier runway, ILS-equipped.
  4L/22R: 5,507 ft × 150 ft (1,679 m × 46 m), general aviation and air taxi.
  13L/31R: 5,141 ft × 150 ft (1,567 m × 46 m), general aviation and air taxi. Used as an Emergency Runway for commercial aircraft.
  13R/31L: 3,859 ft × 60 ft (1,176 m × 18 m), light aircraft only. Used as an Emergency Runway for commercial aircraft.
  


4L/22R, however, is apparently not available for use by air carrier aircraft even in an emergency, despite being longer than either of the other two general-aviation runways (and also twice and half again as wide as 13R/31L, the smallest runway at the airport), and the only one of the three to be useable when the winds are blowing along a northeast-southwest axis (the two designated emergency runways are both oriented northwest-southeast).

Why are Midway's two smallest runways available as emergency runways for large aircraft, but not the larger 4L/22R?

EDIT: As @RonBeyer suggests in his comment, one potential reason a runway of sufficient size might be unuseable for large aircraft could be if its bearing strength were insufficient (such that the pavement could collapse under the weight of a heavily-loaded jetliner).  However, to hear it straight from the horse's mouth, 4L/22R is rated for aircraft imposing pavement loads of up to 30 short tons per single wheel, or 60 short tons per two-wheel bogie; although not up to the standards of the airport's two air carrier runways (each rated to 47.5 short tons per single wheel, 82.5 short tons per two-wheel bogie, or 125 short tons per four-wheel bogie) or emergency runway 13L/31R (rated to 40 short tons per single wheel, or 62.5 short tons per two-wheel bogie), it far surpasses the mediocre strength of runway 13R/31L, which is rated for a mere 6.25 short tons per single wheel... and is, nevertheless, a designated air-carrier emergency runway!
Prologue: the quote in the question is from Wikipedia, and the parts there that mention the designation to commercial aircraft emergencies have later been marked as "citation needed". Therefore, for as long as an authoritative source can not be verified, it might well be that in reality there is no such designation.

Actually, should the nature of the emergency require, any runway (or even taxiway) on any airport is available for any aircraft if the crew so wishes. Some more or less famous examples of such situations (Air Safety Network):

Air Canada flight 143 (The Gimli glider, fuel starvation)

United Airlines flight 232 (Sioux City, total loss of hydraulics)

Air Astana flight 1388 (Portugal, reversed aileron cables)

If you are out of fuel or experiencing severe control issues or something of such magnitude, regardless of what you are flying there is no way ATC is going to tell you "sorry you can't land on the 4L, turn to heading..."

This might seem like splitting hairs, but that's how it is. To state that some runway is (or is not) available for emergencies of a certain aircraft type, is somewhat of a moot point.

After you've spoken the three magic words, the ATC has to, and more importantly, they want to do everything they possibly can and more to help you out. You are finally free as a bird, if only you did't have an emergency to deal with...

Speaking of splitting hairs, Gimli glider did not land in an airport at all, but it's the most famous case of the sort...