Pitt can't do the same thing and hope 60K-70K show up so it provides a good gameday atmosphere.
70K is too big for Pitt, too big for TCU, Stanford, Oregon, Texas Tech, etc. There is no reason why Pitt shouldn't close off sections with or without tarping to move people closer together so the place looks and feels more full.
Several posters have shown pictures of other venues that tarp their stadiums/arenas and fill the lower bowl. They all look great and provide a much better game day atmosphere. Not sure of the logistics of tarping and untarping for the steelers game but I cant imagine it would be that hard. LIke you said the problem isnt the size of the fan base as much as it is the size of the stadium.