Divulging your means and methods is a surefire way of getting the cartels to adjust their operating procedure to avoid detection. If they're being watched via satellites, they'll move their ports. If their cell phones have been hacked and are leaking info, they'll start using different devices. If there's a human source giving the info, he'll be hunted down.

It's not like this is the first time when the US has withheld evidence for certain actions it took. That doesn't mean the evidence isn't there. This is generally a problem with judging government behavior due to the information asymmetry