We refuse to show emotional vulnerability because it can be taken advantage of (even by otherwise good women who in moments of rage/argument will go for the "low blow") and come off as extremely unattractive/weak as being affectected by our situation emotionally instead of enduring shows lack of mental fortitude. In my experience as well as that of my friends, many women get the "ick" subconsciously and involuntarily when such displays of emotional vulnerability are made.
I don't even think necessarily these women are necessarily bad as the "ick" isn't even something they can control, it's just the reality that *many* women in relationships have such an "ick" and we are not willing to test to figure out if you have it. I have even seen women, by their own admission, getting the ick when their boyfriends cry even AFTER they had explicitly asked their boyfriend to be emotionally vulnerable, leading them to express being conflicted and even shameful of their "ick".
These societal standards have been built on the back of biological-emotional realities of partner selection and attraction, things with roots as these are not something to be tampered with and are best left as is. Whether or not it is "fair" or "too hard on men" is entirely irrelevant, it is simply how relationships and socialization function.