They're holding back for the sake of their release schedule. They are, once again, anticipating drought so while we have a wealth (lol Zelda) of games at launch we don't have a ton of VC games. When Wii Play and WarioWare are done, and Nintendo has nothing to release in April, we'll see a flood of VC games.
I don't know how many times I've said it, but this is dumb. Why not coordinate with third parties to ensure a solid release schedule over 2007? There are hundreds and hundreds (thousands?) of VC games that could be out, why not start releasing a crapton of them NOW? If I haven't downloaded a few of them by the time a drought hits, guess what, I'll download more (and play my PS2, God of War II coming out soon). Not to mention that not every Wii gamer is going to be using VC, so a sparse release schedule just screws them all the more and VC games won't band-aid the problem. Nintendo's setting themselves up to burn people just like they did on GameCube if they are anticipating there won't be any games to play.
Nintendo believes they are the only ones making worthwhile games, ergo making sure there's either a Nintendo-developed Wii game or VC title out there is a safety measure. They just can't trust third parties to do their work for them (perhaps rightly so,) but that's the strategy Sony has been relying on for years. Court third parties, release tons of VC games, everyone wins. Anticipating droughts and sprinkling a few VC games when they happen is not the Wii strategy they need.