All of the big 6 clubs could have challenged for the title if they figured out, or stumbled across, early on, a line-up that works in terms of getting results, then they enjoyed good fortune with regards to avoiding injuries and suspensions, crucial decisions during matches went their way, kept mistakes to a minimum and timed them so they didn't cost the team points.
Chelsea's title win under Conte is a classic example of this. They got a lot of points with what appeared to be an unspectacular line-up, though clearly Conte's tactics caught out many opponents. Leicester's title win under Ranieri was achieved with far fewer points but the same principles apply, unchanged line-up most matches, players understand their roles, work hard for the team.
Mourinho managed to confuse himself, and more importantly, his players, by bringing in Sanchez then Fred, then forever changing his line-up and formation. There were also issues regarding our lack of work-rate, too many players that don't cover enough ground over 90 mins. The players cost us points too this season with far too many mistakes and immature performances.
Solskjaer has a clear first choice line-up. We don't yet know how well the team will respond to various match situations. I think burn out from the World Cup would have been a danger regardless of who happened to be in charge.