Why is this the answer?

The answer is walnut because this clever riddle takes you on a special journey to explore all the different layers of a walnut as it grows on the tree. It is a wonderful puzzle that uses word pictures for each part of the nut. Let us start with the first set of clues: "As soft as silk, as white as milk." This is the part of the riddle describing the delicious kernel, or nutmeat, that you love to eat. When a walnut is cracked open, the food inside is a light color, very much like milk, and it is soft and pleasant to chew. To call it "soft as silk" is a beautiful way of describing its smooth texture. This is the treasure hidden at the center of the riddle. The middle clue, "as bitter as gall," talks about another important layer. This part is a bit tricky, because it refers to the thin, wrinkly skin that covers the kernel. Right before you get to the wonderful soft part, there is a very fine layer of skin that you sometimes peel away. This skin has a very strong, not-so-sweet taste, which the riddle describes as bitter. It is like a second layer of protection for the best part. Finally, the last two clues tell us about the walnut when it is still hanging on the branch: "a thick green wall, and a green coat covers me all." When walnuts are first growing, they are not the hard, brown shells you buy at the store. They are completely encased in a huge, round, solid green casing called a hull. This green hull is tough and thick, protecting the soft food inside from the sun and any little critters. This "green coat" is the outermost layer, which eventually dries up and falls off when the walnut is ready to be harvested. So, the riddle cleverly walks us through the entire life of the walnut, from its strong green shield to its sweet, milky heart. Every clue works together to point directly to the mighty walnut.