I have a hunch that exposives detection is so important that this activity has been moved into the "black" budget of CIA/NSA and MI6, and thus DHS doesn't need the budget for this activity. I don't have any inside information; that's just an intuitive hunch.
I didn't complete that statement: In other words, I have a hunch that CIA/NSA, MI6/MI5 are now responsible for explosives detection in airplanes and key locations throughout the world, and these covert agencies are now handling this activity.