Because the notion of masculinity is very vague.
I was just thinking that.
Sometimes I think there's a subconscious attitude for many that masculinity is negative, as well.
Its strange... its pushed for so many men. Then when they behave in the way they were pushed to, they're sometimes painted in a negative light. How confusing! We aren't so hard on women who act masculine, though(actually, this is often encouraged).
In the end, it seems we(speaking of pronounced social attitudes) want women to be masculine, and men to be feminine...
Eh, I hope there's a day we can all just be who we are without pressure to act differently(in absence of violent or harmful tendencies, of course).