He did one wrong act and he changed the course of History. From here we see the importance of a single act. To us it may not seem significant but in the eyes of Hashem, it is very important. Yissacher and Zevulan are never mentioned except when they can blessed by Yaacov and later on by Moshe and also by Korbanos and Flags. Why each one has the tests he has is a Chok.
Psychology has an excuse why every person didnt make it.
Too much responsiblity. The Middle child is always neglected. Thats good for emptying out your pocket on someones couch. But in Torah it's only an excuse. So it is with all HAshem gives us.