Because he is the god, the supreme commander of everything.
He is here to preserve Dharma, not fight it.
Everyone knows that he is the avatar of Vishnu and that he can finish the war in one stroke of his Sudarshan Chakra.
The God can finish the war but he chooses not to. Why? Because if the God helps mankind, how will mankind learn? Mankind must learn on its own.
If I help you achieve Dharma, mankind will never learn anything. I can kill Bheesma, Drona and all this maharathis in one blink of the eye. But I won’t. Because I am not the guardian of Dharma Parth. You are the guardian. Itihas will look up to you, not me. Itihaas will look up to Bheema, not me. Draupadi’s hair will look up to you, not me.
Hence, hey Parth. Wake up. Be brave. Fight. For that is your dharma. Don’t ask for my help. There hasn’t been a time when I was not there and there won’t be a time when I will not be there.
But that doesn’t mean that warriors like you will come up to me for help and lure away from Dharma. Mankind is looking towards you for an example, don’t disappoint human spirit. You are nothing but an instrument for Dharma, so just do your thing.
And how dare you tell me to fight the war on your behalf? Have you forgotten the humiliation of Draupadi? Have you forgotten how you were cheated time and again and humiliated for what you deserve? Don’t you have any concern for the powerless people of Hastinapura, they are looking towards you in hope. Don’t you have any self respect, you are considering these adharmiks as your “relatives” and telling me to find a solution since you are afraid to shed blood of your teacher, your brothers and cousins.
I won’t help you. This is your war and you have to fight it.
~ Krishna to Arjuna in the battlefield