Orion drive is wasting energy?

The Orion drive idea of dropping nuclear bombs out the back, the blast hits a big plate to push the ship forward. But only a small amount of the bomb is actually propelling the ship. But why not make a nozzle on the engine instead of wasting so much energy. I have a few ideas why they haven't done that, one would be that the nozzle would be destroyed. But then cant the nukes be smaller? I would think an engine like this or Small 'Nozzled' Orion drive would be more efficient?