I work at a Blockbusters in Manchester, and we don't rent Wii games - we didn't rent GameCube games either when they were available.
Both games and movies that are bought for the purpose of renting cost more than just a copy for personal use. The reason is that they are licensed from the vendor for distribution - and it's that license that costs more than the normal retail sticker price.
Right now, it's not worth Blockbuster's money to buy a ton of Wii games that won't get rented. And it's not worth Nintendo's time to go through the licensing scheme with Blockbuster since people are still buying games like mad.
Once the user base gets to a certain point, it can be assumed that people will be typically borrowing games before they buy them to try them out. Until that point is reached, it's absolutely in Nintendo's best interest to not offer games for rental to increase sales.
It's also in all of our best interest since higher game sales means more third party developers will dev for the Wii and we get more and better games.