Final answer:
Macbeth obtained the title Thane of Cawdor because the original Thane of Cawdor was a traitor.
Step-by-step explanation:
Macbeth obtained the title Thane of Cawdor because the original Thane of Cawdor was a traitor. In the play, it is revealed that the original Thane of Cawdor was involved in a rebellion against King Duncan. As a result of his betrayal, the king ordered for the execution of the traitor and bestowed the title of Thane of Cawdor upon Macbeth. This event takes place early in the play and sets the stage for Macbeth's ambition and descent into darkness.