what the city fans wrote related to good relationship with Madrid is right. There is some kind of non aggression pact (with Milan as well) between the clubs,was done with Soriano basically to share information about footballers agents,false articles in press,annoy Barcelona,stop PSG,etc.Maybe for that reason Madrid never tried to sign Kun or Silva despite the rumours.Now the press is talking about Madrid paying 90 mill € for Kun,could be a way to get a galactico transfer while helping City with FFP.
Anyway the big reason is commercial.Florentino smells the money and has been trying to build a Madrid theme park in Dubai or Abu Dhabi years ago,besides maybe Mansour petrol company could fund new Bernabeu(Abu Dabi Bernabeu
)