It's very similar to the case where an otherwise fine candidate was turned away because he was too tall for fast jets, but not too tall for other aircraft. Where's the sense in that?
Whilst overall height is a factor, the main reason for taller persons being turned down for fast jet aircrew is due to the length of the thigh from the buttocks to the knee.
The reason for this is that the cockpit environment is quite restricted and long thighs could extend the knees beneath the instrument panel. Ejecting from a fast jet in such circumstances would probably result in a searing pain just above the knees, followed by a dramatic reduction in height; seriously though, it would have fatal consequences.
I wouldn't think it would be practical to consider changing the design of military strike aircraft to accommodate taller persons..or would it? Any ideas?
This is the simple explanation - I'm sure there are those far more qualified than I to give an in depth version, if required.