Why Macbeth and not Banquo? There is no question that, from the very beginning of the play, the witches are after Macbeth. But, if you follow their words, they want to ensnare Lady Macbeth, too. Banquo is not on their menu.
To the witches, Banquo is small change. If they tempt him to murder the king, what have they gained? He's not much of a prize. First, although Banquo was a brave warrior, like Macbeth, he was not quite as vital to Scotland's victory as was Macbeth. He is not nearly as pumped up with pride. And Banquo is a simpler man with less ambition than Macbeth. Perhaps this is merely because of his sanguine temperment but maybe also because he has a young son, Fleance, whom he loves and cares for. Banquo is innocent and loving by nature and just doesn't seem ripe enough, or ambitious enough, for picking.
Macbeth, however, was valient and viscious in the war that is being fought as the play begins, and he is held to be the one who is most responsible for the great conquest. Also, he is cousin to King Duncan. Both of these facts make him a far greater prize for the witches' mischief if they can tempt him to murder the King.
Now, not only does Macbeth have lots more to lose as he is riding so high when the play begins, he also has a very ambitious wife. We know that the witches have already begun to infect Macbeth's mind early on in the play with their mumbo jumbo; his very first words in the play (Act 1 scene 3):
So foul and fair a day I have not seen.
echo the witches' words in the very first scene:
Fair is foul, and foul is fair:
Hover through the fog and filthy air.
And we also know that the witches have visited Lady Macbeth, too, because of their snide reference to her (Act 1, scene2):
A sailor's wife had chestnuts in her lap,
And munch'd, and munch'd, and munch'd:--
'Give me,' quoth I:
'Aroint thee, witch!' the rump-fed ronyon cries.
Her husband's to Aleppo gone, master o' the Tiger:
But in a sieve I'll thither sail,
And, like a rat without a tail,
I'll do, I'll do, and I'll do.
Oh yes, the witches are after two people, but Banquo's not one of them.
Although Macbeth is obviously the focus of the witches' attentions, Banquo is offered prophesies as equally vague and inviting as those given to Macbeth -
In fact, even Macbeth sees that Banquo's prophesies are more valuable and have more longevity than his own: this is his motivation to kill Banquo and Fleance. The reason that Macbeth is much better sport for the witches is that they find him easier to draw into evil deeds. Banquo is content to let fate take its course regarding his prophesies: Macbeth takes his fate into his own hands.
I don't think that Banquo can be put in the same position as Macbeth though there is little doubt that Banquo was also tempted by the witches. In the opening scene, the 'weird sisters' agreed to meet Macbeth on the heath, and there was no mention of Banquo or anybody else. Again, in the opening section of act1 sc.3, the First Witch told the story of a sailor and his wife, thus making an oblique reference to Macbeth and Lady Macbeth. They were still prepared to meet Macbeth only.
However, the witches also prophesied for Banquo in a language of equivocation:
First Witch: Lesser than Macbeth, and greater.
Second Witch: Not so happy, yet much happier.
Third Witch: Thou shalt get kings, though thou be none.
The witches made Macbeth their primary target. Banquo, incidentally accompanying Macbeth, fell into the trap of 'supernatural soliciting' by urging the witches--'Speak then to me'--as if in a bid to contest their occult knowledge--'If you can look into the seeds of time'.
Banquo, drawn as a foil to Macbeth, is a victim of temptation; hopeful about the future kingship of his son, Banquo does not oppose Macbeth; he rather plays safe and gets capitally penalised for his deliberate compromise with evil.