I can understand why Ezekiel would want to keep it a secret. He probably wants Sam's body, and he wants to be strong enough to stay-put when the news is broken to Sam.
As for why Dean would keep the secret from Castiel: that one doesn't make much sense to me. He really should be getting advice from Castiel right now. I'm hoping we'll find out that he's been having secret conversations with Castiel and that they're both planning for the worst.