Howling:When an actor leaves the role of the doctor, why isn't it kept a secret?

Why is it that when an actor leaves the role of the doctor, everybody finds out. Why can't the actor and the rest of the crew keep it a secret? This takes away the magic of the situation and the actual surprise as throughout that whole episode you know something is going to kill him. I liked the ninth doctors regeneration as I was a shocked as rose was in that scene. With david tenant and now matt smith we knew months before the actual change whereas Christopher eclestons departure was revealed apparently around episode 3's broadcast but it was still less of a major thing whereas with the other two everybody interviewed them about it and their successor. Why can't we have that surprise back? Whats also annoying is that everybody is asking every doctor who related person who is playing the 12th doctor and when they don't tell, they ask who they would like. Even if the crew did know, they wouldn't say. Why cant these idiots just be patient and wait!