It's a combination of having the most talent, depth on the squad, and not catching the injury bug during important stretches of the season. Having depth and talent are most important though because if a central player goes down you can always have someone fill in right away and production should not drop off. As far as talent goes, to compete in the EPL, you need it. That's why over the next few years the only true contenders for trophy are City, Chelsea, Man U, and Spurs and Arsenal would have outside shots. That's why as an American I've always been confused as too why leagues in Europe do not implement as salary cap system. Money pretty much runs all the leagues of the world, whoever has the most of it fields an immense squad that the lower teams just cannot compete with over a full season.