Because our enemies know this stuff. It is only kept quiet to keep the sheeple passive.
Yep. If they really wanted to destroy america they could move here illegally and just start working here, take over our economy and...wait, isn't mexico doing that already??? With a billion plus muslims in the world, they could spare a few hundred million or so.
TLR
The terrorists don't know whether the radiation detectors are effective or not, what fraction of the cargo is actually inspected, etc.
To keep them not knowing it is a deterrent.
To go and tell them, as this open session in the House did, and the NYT reported it, IS GIVING them concrete information they can use to avoid detection and plan a catastrophic attack upon us.