Journalistic preference aside, there is another and (I believe) far more important reason for the over-estimation of the power of presidential communications.
The emphasis on communications offers a very handy and acceptable excuse to presidents and their top aides.
As political communication has become more professionalized, politicians—and especially presidents—have come to see communication as something external to themselves. If you tell a president that he or she has a "communications problem," he or she hears that somebody else has failed.
The president's decisions remain as correct as ever. It is only the minions who were supposed to explain that decision that have let down the team.
It is not embarrassing to the president to order them to do better work. It is not embarrassing to replace them. But to replace the economic team—to revise unpopular policy or repeal counter-productive legislation—to withdraw from an unsuccessful war: those things are embarrassing.
So they tend not to get done until the last extremity. A presidential speech, however, can be given at the first sign of trouble. It is the classic quantum to plug into the classic bureaucratic variable:
Something must be done.
This is something.
Let's do this.