There are so many uses of flour that many different characteristics are needed. Different wheat varieties have different amounts of protein so the flour made from them has different uses. eg to make bread you need a very high protein flour (known as "strong"). Cakes biscuits and pasta need less protein to give the structure we expect in the finished product.
"All purpose flour" has always confused me as well, I've always assumed its what we call "plain" in the UK.

I have also noticed that the price has gone up enormously recently.