The simple answer is that mattress manufacturers stopped making mattresses you can flip because they began making one-sided mattresses. One-sided mattresses have a Pillow Top on one side only. They cannot be converted because there is nothing to sleep on the other side except the springs.
Most people believe that flipping their mattress will make it last longer. Although this may be true to some extent, it is not the main reason why mattress manufacturers started making one-sided mattresses. The main reason is that Pillow Tops wear out faster when flipped. This is because the top of the bed is exposed to more wear and tear when converted.