It wasn't the yell but how Dean pulled off the yell. If it didn't seem so un-aesthetic it would not have cost him. Hillary knows how to aesthetically cry. She practiced for months in front of a mirror to perfect it.
A key gauge American's use to choose their leaders is how aesthetic the candidates display emotion. This is done at a unconscious level, as the apes we evolved from picked their leaders with the criteria. You simply can't have your leader, in the middle an attack by a neighboring tribe of gorillas have an emotional outburst that doesn't fit within the tribal expectation for such a thing. It could seriously impact the survivability of the tribe. On the other hand, properly formatted emotion can really bond the tribe, resulting in much better poo flinging.