Why have men been historically discouraged from embracing vulnerability?

part of that mentality originates from the military.

"been" discouraged? Or discouraged themselves? (and their sons, possibly) I guess men have traditionally been the protectors, the warriors, probably for no other reason than they were stronger, in general, than women. So, "big strong men" can't exactly go around crying. But strong men do cry, and do make themselves vulnerable emotionally, in times of peace especially.

It comes from back in the caveman day.

It's all those women! They won't let us cry! They want a REAL man! Humph!