It is somewhat hard for devs to turn on a dime. They can't just stop working on a $20 million project that they've already sunk $15 million in just because they suddenly realized they put it on the system with the lowest install base by far. The next group of games will see much bigger budgets for the Wii since devs won't make the same mistake twice.
As far as the Wii is concerned, it is undoutedly easy to develop for in general but not at first. It is essentially the same architecture as the GameCube (which was also easy) but how many third party devs took the time to fully utilize the GCN's hardware? Not many, RE4 is one of only a few games that used even half the GCN's power. One of the primary problems being that the TEV on the flipper, which could produce the same type of effects the Xbox's shaders did,was little understood and somewhat complicated. Presumably the Wii shares similar issues with its GPU. Even something easy will take time to learn.
3rd party devs are probably also spending more time on the controls than usual (even though it is hard to tell they spent any time when playing some of their games). One of the reasons being that until a few months ago devs had to figure out the calculus functiones required for each move they wanted to capture. It was taking a lot of time and money to do that. Now they not only have more experience with what works and what doesn't but there is also a computer program that automatically generates the function based on how the programmer moves a Wiimote. Why Nintendo didn't have such a program over a year ago for 3rd parties is beyond me.
It is somewhat forgivable that the 3rd party devs aren't ready. First year games from them are usually bad. The Wii is certainly worse than most consoles since the companies stupidly missed the boat and the new controls took more time than expected. These companies have and are paying the price for it, their earnings have been disappointing while their crapware isn't selling and it's all they have. With time and experience their games should markedly improve on the Wii. I'm sure they don't want to to face their shareholders with poor earnings again next year so they really don't have a choice in the matter.
