Comments are often highlighted discreetly. So the highlighters make the choice that comments are less relevant than code.

About the success/failure associated with the red/green colors, I admit that this association should not be made. I suppose Github uses these colors to match the red and green on the other parts of the website. I find blue/olive is pertinent as alternative. However green/red is associated to creation and destruction too. In this case, this is how I understand the colors. Thus the red and green colors are more pertinent than blue/olive.