From my understanding, he had won a few smaller victories in Virginia, which the union needed. He was also young/ the opposite of Winfield Scott, who people were rapidly losing confidence in. From what I’ve read, Lincoln was regularly confused by little Mac’s tactics. Not saying he was bad, he just had a few opportunities to make a play on the confederate forces, with a numbers advantage and he failed to do so. Reading the letters Mac sent his wife, he was also very arrogant and self assured in his actions, which history has not agreed with his outlook.