The plastic ones are easier to process and recycle than the cardboard cans which will biodegrade faster and easier anyways.It's part of the Green/Environmental policy of more economic recycling and reducing the carbon footprint.
If motor oil is so expensive now that they are charging 2-3 dollars a quart for it why are they still putting it in plastic bottles which are also made from oil? Why are they not going back to the old cardboard cans that used the steel nozzle. :)
The plastic one last longer and are less prone to damage and leakage. They are also more convenient, unscrew the cap, don't look for something to open them with. They also take up less space in shipping. Less packaging, more can be transported in same volume.