Third party publishers move in a herd, and very few are willing to break free of the "Safety" of the herd even if they're heading to slaughter ... The reality of the market is that (no matter how popular a genre is on a platform) only the top handful of games released for any genre on a particluar platform in a year sell particularly well, and at some point it makes sense to produce that kind of game for a platform where the genre isn't as popular than to produce another game on a platform where it is popular.
With shooters (as an example) the top 3 or 4 released for HD consoles in a year have demonstrated really strong sales, but beyond that the sales are no better than what (the often third rate) shooters on the Wii have sold; and with (often) 10 to 15 first person shooters being released on the HD consoles, 5 to 10 of them will face the high development costs of HD consoles without being able to see sales that are better than (the often third rate) shooters on the Wii.
Now there is some truth to many developers wanting to focus on higher performance systems, but at the end of the day publishers dictate what games are produced to which platforms; and developers have (in the past) switched platforms of what they were proposing to publishers because the publisher was looking for games for another platform (even if it means that the developer doesn't get to play with the latest and greatest technology).
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