The way I see it is that for most 3rd party publishers the PS3/360 combo is effectively a single platform that they can develop the same game for thus most games that are multi-platform are on those 2 consoles.
The HD consoles are similar in terms of performance and controls so consequently it makes sense for most 3rd parties to take this approach. It is very telling to note for example that while the latest iteration of call of duty has sold millions on the 360 and PS3 it is still shy of a million on a console with an install base nearing 50 million.
With the possible exceptions of a couple of publishers like Sega and Capcom not many 3rd parties are having success on the console. It may be argued that they haven't invested enough money and effort into the wii but the fact remains that it remains far more attractive to develop multi platform games for the PS3/360 than it is for the wii at this time.
Third parties may well support the wii more in the future but as long as the HD consoles are a viable alternative it would seem that the 3rd parties will remain content with the status quo IMHO.
