It's because of the way they set up in big games. Mourinho is too conservative, and I don't blame him... he's had a good track record with his style. But when all the other big teams are playing football that's good to watch, it's less easy to justify. Honestly I think his side is too imperfect compared to his Chelsea and Inter to be able to defend like that and stay fucking deadly on the counter. He's also got players at his disposal that are better suited for less rigid approaches. And when they're playing us and not testing our wonky defense, sure we might not be able to score... but they're also not exploiting our weaknesses.
I hope to fuck his collapse at Chelsea and the rise of sides playing entertaining football is an indication that Pullis' and Porto Pulis' style have been sussed out.