I agree, it isn't because men aren't emotional and (as a generalisation only) are less in contact with their emotions, but they aren't allowed the same development as women are. I reckon it's to do with the relationship with their mothers that takes a sharp turn when they reach puberty and are socially required to model their bearing on other males, the father ideally.

Men hold back a lot more because until pretty recently, it wasn't done to show their emotions.

As I see it.