I remember talking to my grandfather many, many years ago (he died in 1996) about the loss of heritage buildings. Although he wasn't from Calgary (he had been a laborer on the Glenmore Dam), he had great insight into the mindset of the 1950's and 1960's. Calgary boomed spectacularly from around 1900 to 1914, when most of these buildings went up. The city then stagnated until around 1950. Many of those buildings fell into substantial disrepair during the Depression. Sandstone is of particularly high maintenance given Calgary's abundant freeze, thaw cycles. All of the western world was more than eager to put bad memories behind it, but Calgary even more so as its downturn was more severe than that suffered by almost any other North American city. The general feeling was that Calgary 1.0 had failed and the 1950's was a chance to start over.
Another old timer I met in BC once told me that the biggest tragedy of Calgary's clean slate approach to redevelopment was that it lost what was probably one of the most unique (albeit small) downtowns in North America, with its railway inspired architecture and sandstone construction.