I think that Nintendo doesn't want to change their internal development structure to account for demos. Early demos are basically rushed playable portions of incomplete games, which.... takes extra work and time and money which the dev team may have to throw away. When E3 finally heaved a great big sigh and collapsed in on itself due to its own weight, I remember reading how some developers were happy that they didn't need to waste resources making a snazzy, but fake, demo of their half-done title.
Now, demos of games that are already out, or recently released, on the other hand, could make sense. Could developers cut games down to a one level trial after they're done for demo disc purposes? And would the extra expense, and distribution costs that manufacturing demo discs would accrue for Nintendo and/or publishers, be justified by any resulting long-tail sales?
Still, at the very least Nintendo needs some packed-in demo video reel at least. They had those video demo discs for the GameCube, why not revive that program, cut out the playables, and just distribute those preview videeos and trailers willy nilly?