Because the effectiveness of City's style of play requires every player to approach every game like a maniac on speed, and it is a physical and mental near impossibility for a team to do that for a whole season, or even a majority of it. It showed very clearly last season when they couldn't keep it up for more than a couple of months...the results followed suit as soon as Spurs burst their confidence/belief bubble. It shows already this season for me. Against Arsenal they looked really laboured and I've not seen them get that intensity back since.
When the intensity drops, it becomes easier for teams to organise against them, and to counter attack...and City leave themselves so open, that even if they have 60%+ of possession, if they are letting teams counter them, they are going to concede chances. There isn't anything there for them to fall back on. They play the way they play and it either works and they control the game, or they struggle.
The way us and Chelsea play is very different. United or Chelsea can win a game by letting the other team play and picking them off...or they can press for a goal when needed. There isn't this super intensity one week after another. There is a solid defensive unit to fall back on if other areas of the performance aren't quite as good. There's a way to win games and perform to a consistent level over a season that isn't going to run into a brick or fatigue wall at any point.
I'd still make City favourites but I think they'll definitely need the head start they've given themselves, because I don't see them going back to scoring an average of about 5 goals a game, and without that struggles will happen, as they have started to.
