[trhway]

My understanding it is economics, range and time. Technically Concorde of course could fly subsonic. It is just that fuel efficiency was lower due to supersonic design of wings and engines. Its engines were designed with Mach 2 ram effect in mind - that ram pressure on top of mechanical compression resulted in high compression ratio and thus provided high efficiency - and without that additional ram pressure (which is much much lower at subsonic speeds) the pure mechanical compression ratio of its engines was pretty low and thus low efficiency. It naturally didn't have that switch from mechanical compression to pure ram like SR-71 :) The Concorde supersonic wings have low lift-to-drag ratio at low speeds which naturally means high thrust - high fuel consumption and resulting high noise - to keep it in the air at those low speeds.