Why wasn't a scramjet used for the Concorde?
A supersonic ram jet (scramjet) requires a fuel with a very high flame speed, so the combustion doesn't take place after the fuel-air mixture has left the engine. Aviation fuel would be completely unsuitable, and only hydrogen gives acceptable results. The low density of gaseous hydrogen, the storage problems associated with cryogenic hydrogen and the creation of a hydrogen infrastructure would had created tremendous problems for the Concorde development.

A scramjet is like wing sweep: It is best avoided if possible, but necessary if you want to go really fast. For the scramjet the speed above which it makes sense is Mach 4 or 5, and Concorde was designed for Mach 2.