Partly because a lot of game reviewers hate the Wii and 'waggle' so they consistantly give Wii games lower scores.

Partly because (like all reviewers) they base too much on production values and not enough on fun. A lot of Wii games are terrific fun but don't score in the 90's. NSMBWii for example. In the similar way, most people don't see Academy Award winning movies because what critics think is good is not all that fun to watch. Sales is a better reflection of what the average consumer finds 'good'.

Partly because HD systems have more decerning gamers that demand more or at least, pay a whole lot more attention to reviews, so publishers strive for high review scores on HD systems.

Largely because reviews score have absolutely no impact on sales on Wii titles so publishers aren't catering to reviewers on Wii games.

So if you want a game to sell on HD systems, you cater to reviewers tastes, if you want it to sell on Wii you cater to (Wii) gamers tastes.

even worse when you consider that 3 of the 9 wii games above 90 on metacritic are last gen games

edit:4 of 9

A quick personal check:

5/9 of the Wii games are interesting me.
6/19 of the PS3 games are interesting me.
3/22 of the XBox 360 games are interesting me (alle three of them are also on the PS3 list).

Wait- what...?

GET OKAMI NOW!

The Wiis just a kids thing, its cheap to produce games for so you get a ton of crap released.

But I'm a grown up. What should I do?

What's wrong with games with a score in their 80's or 70's? games don't have to be near perfect.

well, the wii does have the first party games you would expect from nintendo, it has a few third party gems, and a few good wiiware games. The only thing it lacks are the blockbuster games.

So what do you think about the effort on the Wii in 2009 compered to the others?

I believe the Wii's library is the weakest in 2009, so it was in 2008. Things might change though