Basically it comes down to this:
Developer A has $15m. They can either develop for the HD consoles or the Wii. They can develop 3 Wii games or 1 HD game for that budget (just an example).
Of course they will spend those $15m anyways because they need to invest. The problem is this: If they spend all their money on 1 HD game the risk is way bigger. If this single title fails on the market (let's say it loses $5m) they have nothing else to make up for it.
On the Wii if one of these 3 titles loses $5m they could still make up for it with a success of the other titles.
Now some people could say "but the chance of a title bombing on Wii are way higher!" But as Nintendo stated the Wii sold the most third party software last year. So in the end you have more 3rd Party sales on the console and can produce a bigger number of titles.
Of course there are other factors playing into this. For example user demographics (if you are a big FPS developer you shouldn't go for a console without FPS fans), the number of 3rd Party titles on the market (if there are three 3rd Party titles on a Nintendo console for one 3rd Party title on an HD console it shouldn't come as a surprise that the Wii sells more 3rd Party software and the risk for your own games is higher), marketing power (bigger developers have more resources for their marketing) etc.
But overall that's the situation. Currently it seems like smaller developers have it easier on the Wii (because they don't have the marketing power and the "budget minimum" is higher on HD consoles) while bigger ones are still slightly in favour of HD consoles.
Edit: I forgot to mention that the maximum loss of a single Wii game with 1/3 of the development cost is (of course) also way lower. So if your title completely flops on the market you won't get hit that hard on the Wii.
