What incentive does a department have to hide an attack they helped prevent? Operational security. Announcing that you've foiled a Bad Guy plot lets the bad gu

What incentive does a department have to hide an attack they helped prevent? Operational security. Announcing that you've foiled a Bad Guy plot lets the bad guys know you're onto them. More to the point, it gives them valuable, actionable information on what parts of their operation you've likely compromised. See also: the Coventry Blitz. Granted, that motive implies a much higher level of coordination between these agencies than TFA suggests exists, but it's among the reasons for these folks to keep mum about having done their job.