Because they failed to make air rank; or were not GD officers when they did. It says!
Wondered about that - in which case they're not up to date - by quite some years. Of recent incumbents, Wordley, McGeown and James all made one-star, and the latter is still serving (so might not have peaked!).
Of course, nowadays (IIRC) all one-star and above are "Common Air Rank List", so are no longer GD ............ so we'll never have GD Air Officers any more
