I think in a lot of ways we completely fail our boys (and thereby our men). It is more than the not crying part, it is the complete lack of teaching them ways of understanding, communicating, and managing the whole range of emotional content human beings feel, whether that's sexuality, pain, anger, fear, etc.
It's a big problem because in the absence of effective tools, we rely upon what we know, which too often is violence or other physical actions (although properly channeled physicality can be very therapeutic). And without the ability to shed negative emotions (which crying in part does) we end up strapping it to our souls. The high rate of suicide by men is a grim example of the problem.
I don't know why we have so stigmatized it. We should view it simply as a physical response to stimuli. Like sweating. Heck, we could consider it mind sweating. It just means our brain had a good workout. I actually kind of think that the need to cry can represent someone who is courageous enough to feel deeply. And that actually takes some strength.