When developers look at the Wii, they look at what games are driving Wii sales. It's stuff like Wii Fit, Wii Sports, Guitar Hero, Super Smash Bros. Brawl, Mario Kart Wii, and Super Mario Galaxy. Those are all established game genres on the Wii, so if you make a game in the same vein as any of those you're pretty much assured that you'll have an audience that's willing to give your game a shot.
If I was a developer of, say, a first-person shooter or an epic RPG (a la Bethesda), I'd look at the Wii and see it as a pretty risky proposition. That's because there's no real history of those genres on Wii (at least nothing notable), meaning that there's no built-in audience for a game of that type. And, as a third-party, is it my responsibility to cultivate that audience? Heck no. I want to act in the best interests of my company, which is to not expose it to putting a game out there on the market that has a legitimate chance of bombing.
Now - as the same first-person shooter or epic RPG developer - when I look at the 360/PS3, there are several very successful epic RPGs and first-person shooters, so I know - know, not suppose, or surmise, or hope - that there is an audience for that type of game on those platforms. There's a lot less risk involved, because again, it's very likely that there's an audience out there that's willing to give my game a shot. It's likely that I will see some return on my investment.
I think this line of thinking is why you see some developers shying away from Wii. It doesn't help that Nintendo itself isn't pushing anything on the console beyond its own existing franchises and Blueoceanware (it's not like they're announcing a new Golden Sun or a cool new first-person shooter; third-parties are their own if they want to go after that audience, and that audience is already on 360/PS3).
Even beyond the first-person shooter and epic RPG genres that I use as examples, are developers that must think it's pointless to go up against Nintendo games head-to-head. I can see a developer saying, "I want to make a kart racer, but why on earth would I want to go head-to-head against Mario Kart Wii? There's no way I'm going to out-Mario Kart Mario Kart."