Why are some paradigm shifts difficult?
Most paradigm shifts are difficult because that is the nature of paradigm shifts. A paradigm shift occurs when people in a given field discard the ideas and rules that had been the basis for their entire way of thinking. For example, in business, this could mean changing from the idea that management needed to be firm, top-down, and authoritarian to the idea that management should be more collaborative.
This sort of fundamental change is never easy. People will have spent their entire working lives thinking in one particular way. This will make it very difficult for them to all of a sudden change their way of thinking and absorb a whole new mindset. They will resist this change because it will feel completely wrong to them.
Because of this aversion to change, paradigm shifts are generally difficult.