It took years to dig up Mount Weather tunnels and fit them with facilities that would allow to survive there. Given time they might be able to restore MW (take out tons of rocks, reinforce the tunnel structure, rebuild air vents, air flirtations systems and other infrastructure ) but they don't have time.. it would be easier\faster to fit\expand Alpha station or some other bunker.
The problem with that bunker was that it was not sealed correctly. The people inside died from radiation. It would take them way longer to try and seal that bunker than it would finding a different way. It was also in pretty bad condition.
188.8.131.52 wrote: What about that bunker Jaha found a couple of episodes ago, it might be usable if they fix it up. Based on the shows location in the DC area, there might be some secret government bunkers in the area.