The GI of white and wheat pasta is considerably lower than just about all other refined grain products because of their unique structure: The glutens (wheat proteins) that bind the flour together densely and give pasta its chewy texture also protect the starch particles from being attacked directly by our digestive machinery. You can ruin that convenient property easily enough, though, by overcooking your pasta.
Note that gluten also binds the flour in bread, which has a high GI. However, the structure of bread is light and airy, with much surface area for enzymes to work, unlike the dense, compact structure of pasta.