The US didn't get involved because they knew it would be expensive and economically unsound.
They did, but overstepped the mark by trying to build a mach 3 design, which actually turned out to be good thing in the end as the problems in building a commercial carrier which could get through the thermal barrier slowed them down long enough so that the low cost market become the established norm rather than the jet set and captains of industry market that originally drove the design for Concorde.
As for complete dominance of the subsonic large transport category how many B52s and KC 135s were bought by the US government and also did not the 747 arise out of the CX-HLS? Despite there being a clause preventing data gained from Government contracts being used commercially this was circumvented in many ways but mainly by the transfer of staff between the divisions of the company.