Tony Martin has been a great time trialist, but he hasn't been considered on par with Cancellara, whose high regard stems from his achievements beyond time trials. Tony Martin hasn't been considered at the same level as Contador or Tom Boonen, each a specialist in a different aspect of cycling.
Tony Martin's talent is undeniable, but it would be a lie to claim that his TT victories gave him the same recognition as Contador's climbing victories or Boonen's classics victories.
I'm not exactly a fan of sprinters, but Cavendish and Kittel have also enjoyed greater status, although not as much as someone who wins a Grand Tour or Paris-Roubaix. However, they are more considered than the great time trialists.
Perhaps what penalizes that discipline is that the cycling world championships didn't exist until 1994. I don't know, but no, Tony Martin has never been held in the same regard as a good climber like Contador or a good classics rider like Boonen, and what gave Cancellara his status were his achievements outside of time trials.
Remco is part of the big-six mainly because of what he has on outside of ITT, otherwise he would be considered a great time trialist like Tony Martin or like Ganna was until Remco appeared. Ganna has never been considered in a BIG. Ganna has always been expected to do more in classics to have that recognition. That's why their San Remo performance last year was so highly regarded.
You may find it unfair, but it´s simply a description of reality.