Because Real are crap, aside from Ronaldo not a single player has performed well, Alonso fouls everyone, Pepe/ramos/Marcelo try to foul everyone, Ozil/Di Maria drift in and out of games, go missing for several entire games then have one decent performance. Modric hasn't gelled with the squad, both from not being played much, from not being what the team needed and from the team playing badly making it hard for someone to come in and look good.
Benzema has the odd game where he's a powerhouse and bullies defenders not unlike Drogba could but with more pace, but most are games where he can't hit a barn door. Callejon is a pretty decent finisher, Higuain has had injuries, scored a fair few, but he was starting early season when the team lost two and drew a couple and I'm not sure if he was dropped or got injured after that.
The team just isn't playing well, there are rumours of massive problems behind the scene, Mourinho having issues with half the team and in most games most of the team play like it really doesn't matter what they do, Ronaldo will still win it for them, and he has done that plenty of times but can't do that every game.
Overall I just don't really rate Di Maria, Ozil, Khedira, Benzema, at best they are inconsistent, they VERY rarely have top notch games and they rarely have a good 90 mins. For me Ozil/Di Maria can look great but are frequently annonymous, they aren't a patch on Iniesta/Fabregas/Xavi, or even as consistent offensively as Barca's fullbacks.
Defensively... Pepe is just a rabid maniac, he and Ramos love to rush out and assault people but often miss and just leave massive gaps. Coentrao and Marcelo are just rubbish defensively and Alonso can't provide any cover except Song style constant freekick conceding.
They lack leaders, spirit, don't work hard, and massively massively massively over rely on Ronaldo to win them games. People really don't seem to realise what an epically brilliant season Ronaldo had last year. People talk about the strength of their bench and point out kaka..... who has been completely turd for 3 years, Callejon, decent but then Benzema hasn't been good and neither are great by any stretch.
They've not been good in Europe, City have been completely turd in Europe and Real could have lost both games to them with Ronaldo pulling out a last minute goal in the win over City. They looked good compared to City...