Here's an interesting suggestion for why coffin nails were once so common... they were apparently used for more than just nailing down the lid. They were, in fact, decorations.
This coffin, from a museum in York, is a replica of a 19th century coffin that was found in a church. It shows how the nails were used as decorations, over a cloth cover. Before French Polishing was invented, coffins oft times were covered in fabric for decoration.