Satan is much more powerful than god.
Let's pretend God and Satan are about even in all their stats and attributes: what if God lost that first attack on him by satan purposely, and let satan have heaven as his own domain? What if God just let's satan do whatever he wants because he just doesn't give a shit about anything but fornicating with his thousands of female angels? What if satan is infinitely more powerful than god, but allows him to live? (he's the real god, and suppressed his actual power before so god wouldn't die.)