Arsenal failed to win the Premier League because they could not maintain their consistency. I think the reason is that they don't really have the proper preparation and they don't have a lot of options to rotate players, so fatigue and injury became a big problem for them. Besides that, Arsenal also don't really have a winning mentality, one important thing that their rival (Man City) has.
Arsenal had failed to maintain their consistency due to injuries and most likely fatigue too as the same players were the ones playing all the time. But I don’t really get what you meant by Arsenal not having a winning mentality. I think they’ve got a winning mentality and that’s why they had been leading the league table for a while.
They had been winning their games and gathered some good points to be a bit ahead. That’s a winning mentality in my book. They had suffered some set back and lost points which led to Manchester City who was hot at their heels overtake them and finally win the league. Manchester City no doubt has a winning mentality and is a superb team.