Probably because you're watching it on your monitor resolution. The 3DS resolution is pretty small, so it won't look that grainy. Also it does say that it's not indicative of the final product.
Plus someone else mentioned earlier that if you compare the final version of Kid Icarus graphics with it's trailers that there's a noticeable difference in quality between the two. --- Official Manectric of the Pokemon X/Y Boards
"The graphics are rather cruddy looking for the 3DS..."
Everything but Resident Evil Revelations and Rhythm Thief and the Emperor's Treasure agree with you completely. Only two good-looking games on the whole fekkin' system. --- http://objection.mrdictionary.net/go.php?n=6096902 http://objection.mrdictionary.net/go.php?n=6103703