Been wondering this for ages, but now Huffpost has come to the rescue and explained it. I do miss it. There was something satisfying about traipsing round that big Tesco store in the middle of the night with hardly any or no customers around and then seeing the receipt timestamped at 03:01 or similar.
www.huffingtonpost.co.uk

People Are Just Realising Why So Many 24-Hour Supermarkets Disappeared, And Huh
I, for one, wish we could go back.