Well, I believe it 's the limit of the material it was built on. The Concorde was built using alumunium alloy, which was the boundary of the technology then. The affect of kinetic energy heating on the fuselage structure restricted the max speed to mach 2.2. (at that speed the air passing thru the fuselage creates friction and thus high temperature of about 250 degrees F !) If it was to go faster, either very heavy stainless steel or extremely expensive titanium alloy would have to be utilized.