The short of it is that Ares dies at the end of the first game.
The game was originally supposed to be a one-off and was never intended to have a sequel, so Kratos kills Ares and becomes the new God of War at the end of the first game. The ending scene even has Kratos on Ares' throne and talks about how he, as the new god of war, is responsible for all sorts of heinous acts throughout history while footage of modern wars like Vietnam flash by.
But the game did so well that the studio execs demanded that they make a sequel, and then another, and another, and another, etc. They had to keep coming up with new gods to kill and ways to up the stakes by casting Kratos off of his throne, so by the third game, Kratos just murders the entire Greek Pantheon and causes the apocalypse. And then they had to figure out where to go from there. The "reboot" was probably one of the smartest things they could've done for the series. Don't have to keep upping the stakes with gods to kill if Kratos just goes to therapy and deals with his issues.