I think I've mentioned this before--I get that the disciples, or any of the people, didn't understand what Jesus was saying. It wouldn't make sense, especially after the welcome Jesus just got coming into Jerusalem. I mean, yes, of course, there were people who didn't like him (see: Pharisees), but it would seem that Jesus was enormously popular. Why would they think that the tide would change, and so quickly at that?
Group think + Satan = Baaaaaaaaad
God likes code. He likes to tell us things we can't always understand. Why is that? I don't know...maybe so we meditate on it and really have to think about things? The wonderful thing about scripture is that it is written down, so it is perfect for analysis. The disciples only heard things, but even then, it must have struck a chord for them to remember it all and write it down decades later. We have to think.
We need God to understand God, on a certain level. Maybe that's the point? To seek Him more?
I don't know. What would have happened if Jesus was more explicit? The disciples still wouldn't understand, maybe. Or maybe they would have tried to prevent things. In books, at least, when a character knows something is coming, they either ignore the warning or try to prevent it and it never works (see: Macbeth). People are probably the same way as in fiction, and Jesus didn't want this to be prevented.
So maybe that is why.