Is there some fundamental (simple?) reason why Concorde's intake thrust contribution is somewhat higher at a significantly lower speed?
It may sound trite, but as I see it the reason why percentage intake (and nozzle) contributions to installed thrust increase with increasing Mach number is just because the engine contribution to the total gets smaller!
For any given technology level there will be a finite limit to the top temperature in the cycle - usually set by turbine entry conditions. OTOH, the temperature of the air delivered to the combustion chambers will increase with increasing Mach number (and design pressure ratio of course). The amount of energy you can supply to the cycle, and therefore the thrust the engine can develop, is set by the gap between these two temperatures. As you increase Mach number this gap narrows, so diminishing the thrust potential.
At some point the engine becomes that apocryphal pump that keeps the intake going and supplies the nozzle.
You can bridge the gap to some extent by using afterburner, but sooner or later the temperature problem will kick in.
I think this fits with the engine share of installed thrust:
Concorde, M 2.0 dry - 8%
SR71, M 2.2 afterburner - 73%
SR71, M 3.0 afterburner - 17%
So I would say that the reason why Concorde's intake contribution to the thrust at M 2.0 was as high as 63% was because the engine contribution was only 8%.
It would be easy, I suspect (for someone who understood the thermodynamics better than I), to concoct a more complicated explanation, but this one satisfies me at least.