The answer given in the Iliad is simple: because Achilles is the son of a goddess and gods play for him. Rough, but honest.
And what if you look at the situation from a slightly different point of view?
What is Achilles? - The ideal machine of death, laying a bloody path to glory. Nothing else simply does not interest him. For the sake of fame, he is ready to store the corpses in stacks, and it does not matter to him whose corpses, enemies or friends. But he is not even cruel, in the sense in which cruel sadists are. Violence does not give him much pleasure, he just does not know pity. The only thing that matters to him is Achilles the Quick-footed And His Glory.
In our time a person of this type would be called a psychopath. At first approximation, psychopaths seem incredibly charismatic. Phenomenal stubbornness in achieving personal goals in combination with a complete lack of pity seems to people with normal psyche something extraordinary. They look mysterious and attractive, they are followed. And it happens that to death.
What is Hector? - A person who has something to lose. Behind him, his wife, child, family, compatriots. He is not at all ideal, he is the same professional killer as the other participants in the massacre, no matter how beautifully it is called - the Trojan War, the Battle for the Right Cause, or Life for Elena the Perfect. But he is a man with his weaknesses and fears, and whatever his methods (and they are often ugly), his aspirations are clear - to protect what is expensive.