The reason for "one bag" is that they are trying to increase throughput through security. Airports like Heathrow don't have a lot of space going spare and installing extra security points would cause (even more) chaos. Of course it doesn't answer why they don't have all the existing points open at all times

Perhaps they just got sick of the people who'd show up with an oversized rollerbag and a suiter and a purse and a laptop and a camera bag and a backpack and a plastic bag full of magazines and then wonder why it takes them 10 minutes for their gear to be scanned especially when they've forgotten the 3 sets of keys, the Indianhead belt buckle, the wallet, the ipod, the camera, the loose change on their person when they went through security the first 5 times