Because they've bought players who are both extremely talented and have very high fitness levels.
Fitness shouldn't be an issue at Premier League level. Most of our squad aren't even inrernationals. They get summers off, national team breaks, etc.
Well, this is a deep and complex subjective topic and I'm sure you'll have reasonable counter arguments, but the first thing I'd argue is that fitness is always going to be an issue, otherwise managers wouldn't rotate players at all. The winter break is one of the more controversial topics in the sport for the UK at least, with many top managers in the sport calling for it, and that is entirely about giving our players a rest mid-season to recover and extend fitness levels so they can compete. Another perspective is managers protecting players by rotating them and giving them rest. It's all about the long game here, keeping players fresh for the latter stages of the season when they're likely to get tired quicker.
Second thing I'd argue is that fitness per se isn't the issue, it's fitness + performance/ability levels + team's style of play. Our players are reasonably fit, they're all athletes, but the problem is they're barely PL quality so if they're not at 100% and an opposing PL team are then it's handing them an advantage (e.g. we'll tire in more over the course of the match, which means more mistakes, more attacking opportunities missed due to being knackered, etc etc). Whereas Man City's squad is filled with players who in addition to being the cream of the crop as athletes are also top class footballers so even if they're not at 100% they're still likely to be very good.
Third point is that City also dominate possession massively so it's normally the other team chasing shadows and getting knackered both mentally and physically whilst their own players come out of games barely breaking a sweat. By contrast in a team like ours where we're utterly shit in possession, we're the ones going to be doing the running, ergo our players need to be fresh to do all that chasing. On top of this they also have a deep enough squad where if key first team players did get injured they'd get by, whereas with ours it's going to be a serious setback, so given that more games = higher risk of injury I can see why Rafa would be cautious.
Another perspective I'd put forward is to look at individuals. E.g. Ritchie and Yedlin, it goes without saying Yedlin's fitness and stamina levels are significantly higher and that he can comfortably go on for 90 minutes consecutively without seeing a visible drop in his performance levels. Whereas Ritchie is more likely to gas out after 60 minutes, so when the ball breaks he'll be even more restricted than normal. In terms of comparing to Man City, if Ritchie had the pace and stamina of Yedlin and then on top of that had better dribbling/crossing/shooting abilities, you now have a Man City player (basically Sane). You can field a player like that 2-3 times in a week without worrying about their performance levels dropping too much if they get a bit tired because even if they do they're still going to be pretty good. Add in the aforementioned team domination dimension and it's less of an issue for them to have to rotate.
I'm sure if Rafa really wanted to he could field our first team every week till the end of the season in the hope that progress made now leads to success towards the end of it, but it's at the risk of struggling badly in the latter stages of the season when injuries and fitness issues are likely to be more prevalent. I suspect a part of why we saw moderate success in the latter half of the last two seasons is because Rafa has rotated players and kept them fresh.
Top post.
Loudest the Chelsea fans have been all game.