Why does duncan choose to give the thane of cawdor to Macbeth?

In a general way, because he had done good military service to the kingdom. In this respect, he might have given the title equally to Banquo. The fact that Macbeth was of royal blood and a close relative to the king was probably a factor. Duncan was an incurable nepotist as witness his making the feckless and militarily useless Malcolm the Prince of Cumberland.

Do not make the mistake of thinking that it was a reward for capturing the traitor Cawdor. Macbeth didn't do that, because if he had, he would never have said shortly afterwards "The Thane of Cawdor lives, a prosperous gentleman! Why do you dress me in borrowed robes?" If he had said that after capturing him Ross would have replied "Because you just finished capturing him on the battlefield you stupid twit!"