A book I have (Boeing, Planemaker to the World) includes how many orders and planes received of all of Boeing's major models as of the date it was printed. It said the 2707 (Boeing's failed SST) got 125 orders. That seems a lot, compared to the very small amount of concordes and TU-144s that were ever made. What made the 2707 so superior that 125 of them were ordered?
The plane was designed for a US funded competition which it won, after it would get 1 million dollars in funding for further development which automatically means that the 2707 would be the cheaper plane to buy and that it had the US Government Stamp of Approval. This also meant that there were political reasons to commit to buy one of them.
The reason it won that competition is because it was slightly less loud, slightly more efficient and more technologically advanced than its competition.
Never to have been built could have a lot to do with that superiority. All designs start with high hopes and great specs, and only later, when reality interfers, the numbers have to be revised, mostly downwards.
The other factor is size. Being much larger, it could fly more efficiently than the smaller Concorde and Tu-144.