Here are few possibilities.
The first possible reason would be to allow the imagination of the viewer to envision just how Macbeth may have assassinated Kind Duncan, and maybe it would have been too bloody for the viewers of his time, for Macbeth is a butcher, and we know that from the description of the manner in which assassinates Macdonwald; he splits the body of the traitor Macdonwald in half and then beheads him.
The second possible reason would be to place the focus of entire murder scene not on the deed but rather on how Macbeth and Lady Macbeth react to this most heinous crime, for the scene is most nerve wracking in. And Shakespeare really does an excellent job with it. ?The dialogue between the Macbeths is a series of very short lines, some of them are one word lines, but what is said by them afterwards really develops their character.
Yet another possibility could be that Shakespeare wanted to maintain the pace of the play; the play is his shortest, and it moves rather quickly, so by omitting the murder scene the play keeps its pace.
Anyway you look at it, one thing is for certain, Shakespeare did not think that the murder scene was important to put into the play. And you know, the play works wonderfully without it. Doesn't it?