Blessed are those who mourn for they will be comforted. This won't work for princes because if they mourn then the whole kingdom worries. If at war and losing, a prince cannot show his feelings for fear of the whole army quitting because their leader is scared. Without showing feelings, the prince has full control because he is considered a strong leader.
Blessed are the pure in heart, for they will see God. Without corruption, a prince cannot run his kingdom. There has to be some form of deceit because without it certain actions will not work. Like if a kingdom was on the verge of going bankrupt, the prince could not say this because the people would move and leave him with the problem of gaining more money.
Blessed are the peacemakers, for they will be called sons of God. Peacemakers just don't work in royalty and running a kingdom. There is always a hunger for more land to rule over, which means war is needed to take over the land that is wanted. Peace doesn't work since the land wanted would not feel a need to fight for their land because the people who want it are too soft to take it over by force.