The original question was 'why is a box more threatening than a bag?'
In an environment where security is sensitive and where there are well trained people operating it, the key to making it work is seeing the things that don't fit in where they should be and doing something about it.
Try these:
Why is a passenger in a winter coat more threatening than one in a t-shirt (in the summer)?
Why is a passenger without any luggage more threatening than one with a suit case (on an international flight)?
Why is a walking stick more threatening than an umbrella (in the hands of a young able bodied passenger)?
Boxes are treated differently in London for all of the reasons given here (and more). If a box turns up at the loading point amongst suitcases in London it is out of place and needs investigating. First - How did it get there? Second - Has it been screened? Third - What is in it?
Talking to the box doesn't get much of an answer and unless 100% sure it is probably sensible to treat it as suspicious and put it back through the system once some straightforward questions have been answered. It will need to be treated carefully until it's contents are known and it may take a little time to process it again. We all know that baggage only goes on the aircraft in the last few minutes of a turnaround and if taking the box and it's owner away and dealing with the problem is going to take more than a few minutes, it is a simple commercial decision to send the flight on its way without either.
All seems straightforward to me.