I assumed it was the way it was depicted in the EU. (Up until all the Palpatine clones, of course.)
It was a major blow to the Empire, all their leaders were dead, propaganda disaster, they lose Coruscant, etc.
In other words, the conflict is no longer exciting enough to depict in the movies, because the Empire is basically waiting to be mopped up or get a new mastermind leader (like Thrawn.)
Darth Vader getting on the PA system and telling everyone that it's over seems a little redundant, not to mention abrupt with all the emotional payoff going on.