I think it's a combination of things. Definitely the type of fan base is probably quieter than most arenas, with a higher amount of business related tickets, and Calgary having a high percentage of people from other cities in Canada who route for other teams. The last game I went to was against the Rangers. Half the people around me were fans of other teams like the Oilers, Leafs, Bruins, etc..
The building's roof is part of it. Buildings with flat roofs of metal or qdeck tend to have more echo...at least that's my experience with some of the older buildings (The old Pacific Coliseum, Philadelphia Spectrum, Northlands Coliseum and the Forum in Inglewood). Even the slightest bit of fan noise can sound quite loud when you get a decent echo going.
