Think the cleaning out the squad of poor performing senior players with a poor attitude and replacing them with younger, better players with a good attitude is the key reason. You could not win consistently with the squad we had. Just too many poor players in clicks like the Mustafi, Özil, Kola one e.g.Agree with the above, consistency is generally a big one. I also think recruting well and not only looking at player's talent but their character and ambition i.e. players that wil not sulk and will work hard to regain their place.
He cleaned out the senior players in the squad who were influencing the e
poor culture when he first arrived i.e. the mustafi/Özil/kola clique. He gained the respect of younger players, made some of them who displayed the character he desires as leaders and built a better culture around that. Hence, its hard for new players to disrupt that culture.
Now they are replaced by guys like Saka, Rice, Gabriel, Ödegaard etc. etc. Solid, hardworking guys. Could make the list much longer. Arteta has built a squad that will hang in there and fight.
Have faith in the group and Arteta!