Because we never got what MOF and LOS1 set up: a redemption story of an evil man who learns to forgive and love again.
I don't think Gabriel's descent was setup very well in the first, since we were told that by narrator Zobek instead of seeing it through Gabriel's actions, but he was still very sympathetic and likable. It was his motivation, his resolve to stop at nothing to bring back his wife and end the Lords of Shadow, that made his end without his wife tragic. He saved the world but there was no eternal reward waiting for him. If that wasn't enough, he learned of another evil threatening the world, had to sacrifice by his own hand a young vampire girl who befriended him in his despair, and was forced to become a vampire in the process.
Gabriel had the weight of the world on his shoulders for the entirety of the first game, and it amounted to naught for him in the end. No wonder he's angry at the world and God. But this is what's missing in the second game. The only time his motivation for villainy is brought to light is when he's talking with the apparition of his wife, where he boldly exclaims how he will always be a thorn in God's side, but aside from him killing the family in the beginning, and taking out 500,000 Brotherhood knights, there's not much Dracula does to show, rather than tell, that he hates the world and God. This is the opposite of Mirror of Fate. When Trevor is forced to leave his family and is killed, when Simon's mother is murdered, and Dracula threatens both his vampire son and grandson, we could see that Dracula is bitter and doesn't care much for the world, even his own family, for crying out loud. We could tell of his legacy of blood by the way the three main characters talked about him and their motivations.
But since Dracula is not the villain in LOS2 as he was in MOF, there's no payoff for a redemption anyway. Instead, we have an anti-hero Dracula who teeters between a decent man who loves his family and someone who is bad for the sake of being bad. When Gabriel says to Satan how God cares about what's in their hearts, and will forgive them if they ask for it, that should have been the drive behind a redeemed Dracula: when he realizes that he does care about the world, about his family, and about God. Then his motivation to defeat Satan once and for all is clear: not only for revenge, but for eternal justice, to sacrifice what he had so that others wouldn't have to.