Operational need.
The hypersonic bomber died with the XB-71 Valkyrie and the development of effective long range SAM by the Russians. It didn't matter if you were going to fly at M2, 3 or 4 - the SAM would get you.
For FJs it comes down to the environment in which they're going to fight. Analysis shows that the vast majority of their life is subsonic with a small proportion up to M1.3, and a miniscule amount beyond. So that's what the design is optimised for.
Last edited by ORAC; 3rd Sep 2003 at 16:07.