Well, it's because iceberg lettuce is basically only cellulose and water. It has nothing to offer nutritionally.
Now, if you want to facilitate bowel movements and hydration, a little iceberg included in the salad will do that. But if you want to provide some actual nutrition stick to DARK LEAFY greens. Top on the list should be Collards, Mustard greens, Dandelion greens, alfalfa leaves, escarole, water cress, small amounts of many leafy flavoring herbs like cilantro and basil, edible flowers, green beans and peas. Many common garden and lawn "weeds" have a great deal of nutrition to offer like chickweed, goosefoot, curly/sour dock. Before you can harvest them for use you MUST be certain that they are free of herbicide, pesticide and petroleum contaminates. The best part of most other lettuces are the dark green outer leaves that usually get thrown away. Avoid spinach, parsley, chard, cabbage, kale.