I am not an authority (nor should I ever be quoted) but I believe that some of the antagonism against critical rational thought in Sunni (as opposed to Shia) Islam stems from the completely wacky permutations that some people came up with back in the day.

Sunni Islam is basically an orthodox attempt to cross the "t"s and dot the "i"s and answer all the "queries" once and for all.

It also has a lot to do with the rise of political Islam, i.e a bunch of Arab tribesman from the Najd suddenly ruling half the known world and a diverse and unhomogeneous bunch of people.