A number of the players you mentioned couldn't make the step up. Willian wasn't exactly dazzling at Shakhtar before joining; Oscar came from the Brazilian league and played something like 65+ games in his first season, going without a break for 3 seasons IIRC. Salah & De Bruyne were given the chance to mature and grow in the right set up, which was something Mourinho wouldn't allow to happen under his watch. There are several reasons why players fail to produce or improve their game/make the step up, but most of the time, it comes down to the individuals mentality. That sentiment rings especially true with Morata. Everything about his demeanour screams out someone that expects things to magically click/entitled rather than that of a player who wants to better himself to suit the demands of a new league or improve parts of his overall game. I personally think he's nothing more than an impact sub and his previous clubs were aware of that.