There are a lot of reasons, ranging from lack of quality, lack of advertising, failure to market to the correct or total demographic, and lack of an extended demographic to name of few.

Basically, Nintendo makes good games. Some are great, and some are just good, while only a few are below par (Donkey Kong Barrel game, I'm looking at you). Not everyone might like their games (WiiMusic), because they are not in the targeted demographic, but they are quality games nonetheless. Nintendo also advertises many of their games, and they are really good at advertising to the correct crowd (minus that WiiMusic E3 screw up), and many of their games cater to many different demographics, like Mario Kart. These things combined result in high sales for many of their games.

Take out one of these factors, and sales fall, with each factor removed sales falling further. Release a sucky game or a lame port? Potential sales fall. If you don't market your product, potential sales drop, though word of mouth has helped a lot of Wii games. Market a game like Boom Blox to only 8-12 year olds, though many other demographics could/would enjoy it? Yeah, you are loosing potential sales (thank goodness for word of mouth though...). Make a niche title? Get ready for smaller potential sales. That's just how it is. Nintendo is great at making their games fun for everyone, so many of their games are juts always going to outsell 3rd party games because 3rd parties are just not as good in that regard.

I've made a post about this a while ago. Here's a link.

But this is a good post from that thread that basically sums things up:

Well, aside from that game, all the Third Party games that have been made have had nowhere near the effort put into them than a PS3 or XBOX360 title.  Hell, even DS games get more funding and development time given to them tha Wii titles.  If Tird Parties expect to gain more than 10% out of the Wii pie, they need to put more than 10% effort into it.

Cause development on a new platform like the Wii is much easier for the company who came up with it than third parties who have to get used to it.

They shouldn't have to COMPETE against nintendo. You can buy both...

Nintendo does cause some trouble by keeping some of their technology secret until months before its release (motion plus, wii fit board).

But I think in large part, publishers just won't put the effort into putting the big games exclusively on the wii. And until they're exclusive, people will always buy them on the other consoles instead.

nintendo makes good games, most 3rd party devs suck, period.

Nintendo Makes way better games than ANY third party!!!