Because in a Shakespearean tragedy , the tragedy consists in part of the fall of a basically good man through his own 'fatal flaw' - in Macbeth's case, 'vaulting ambition'. His virtue should be clear so that his fall is evident.
Macbeth is truly a tragic hero. A tragic hero is a character who is basically good and honorable, but he has a flaw that leads to his downfall. Macbeth is shown as a brave and honored soldier at the beginning of the play so that we can see his decline as he begins to plot his evil deeds. Macbeth's flaw is his ambition. He wants to be king and decides he will become king at any cost. The killing of the king is the first step in his downfall. By the end of the play, he is killed as a butcher with his head carried on a pole so others can celebrate his death. Readers can see that Macbeth's desire for ambition causes him to be viewed at the end of the play as a man who is totally opposite of how he's portrayed at the beginning of the play.
Ah, good question. From the time of classical Greek theater on, the ideal of the tragedy was that a man of some stature faced a situation that toppled him, evoking, as Aristotle said, fear and pity. Shakespeare's theater didn't hold to that ideal as strictly as some, but in general, the idea was that for a man's fall to matter and have powerful emotional impact, he needed to hold a position of honor and/or favor at the start. To put that another way, who cares if a murderer murders, or a thief steals? That's who they are. But if a good soldier betrays those to whom he should be loyal, that hurts. That matters.