Because these traits are seen as being "weak".
I tell you what's weak; going to war instead of taking the -longer-, -harder-, diplomatic route. Wars, aggression, fighting etc are what we see in the animal kingdom. Peace and negotiation are manifestations of the cortex (what makes us intelligent); fighting, aggression etc are the result of emotions and hormones. NOT thinking. Peace, altruism and harmony set us apart from other animals. And what separates apes, is how they arrange their societies. More harmonious apes are considered to be more advanced than the aggressive, war stricken ones. They are seen as more advanced, because they foster a more efficient, egalitarian society that increases well being.
Unfortunately up until this point, aggression and dominance have claimed monopoly on power and authority, but there are those in history who have made immense changes by going down the diplomatic route. And many of these people have become icons. I think those who are gentle and diplomatic need to bite the bullet and take a stab at getting into positions of power and maybe we will see some further societal changes.
Anyone whose been in a fight knows that it's much easier to yell, insult and curse. What requires more critical thinking, is resolving the issue. It involves stepping out of your ego for 2 minutes and actually acknowledging what the other person is saying because you can appreciate that maybe you are wrong. Maybe they have a point? People think considering other opinions and not being so stringent with what you think is weak. But that is the only way you ever resolve anything. Compromise, coming to the table with an open mind and acknowledging others is the only way to foster progress. You see it in effective business, politics and all forms of diplomacy, really. The reason why we become so stagnant is because we put things that require diplomacy, and such into the "too hard" basket. The way we progress is through negotiation, peace and empathy. And it requires hard work and stepping outside the ego (which is hard for all of us).
But I totally agree with you Izzie.