There are a number of reasons for this ~ some of them are cultural, and some historic, but primarily the biggest reason is the "leadership principle" ~ Shia has a tradition of disaggregated leadership, with authority deriving from a group ~ the religious leadership in fact.
Sunni is a sitting duck for any restatement of the forms the Roman Empire adopted. That includes the exact same "leadership principle" found in fascism and its twin, Nazism, or it's first cousin, Stalinism!
So, what was it you were sayng until you brought up a new topic?
They are functioning in the same manner for the same goals and they DO work together as we have seen in Iraq. There is danger in pretending there is a difference.