Because it is simpler, cheaper and easier to make a square box than a round one (a square box can be formed from a single sheet of card ) and also it is easier to store, transport and pack square boxes which will remain in a flat state until needed and are quickly assembled at point of use. A round box by its very nature needs to be assembled from at the least four sections, or to be pressed to shape using special machinery. It also needs to be in two seperate parts, the container and the lid. Storing round boxes wastes space due to the unused volume inside, and the unused space between each box.
The Ecodesign Strategy Wheel shown below relates a products life cycle to the ecodesign strategies.