Why did dissatisfaction with the authorities and the structure of society take the form of heresies in the Middle Ages?

Because at that time the church played a very important role in the life of the state. We can say that the church was in some way the face of the state. Any unrest and protests against the authorities meant protests against the church. A very convenient mechanism for keeping the current tuning. The leaders of the uprisings were immediately recognized as apostates, blasphemers, antichrists, heretics and others. Thus, the church directed not only the military police against the rebels, but also turned the rest of society against them.