In the bonuses of the long version, he justifies it by the fact that it would have lengthened the movie (and the production) too much for too little interest.
And I must say that I quite liked the "alternate Shire ending" he did, where we see the heroes coming back changed in a world that stayed the same, and which doesn't apprehend what they did, in a stark contrast with the Gondor crowning scene where all Middle-Earth bows to them...