Peripherals need good 1st party support in order to sell.
The EyeToy was, at best, a cute side project for the Sony PS2. So they had a handfull of games then dropped support altogether. 3rd party ignored it altogether.
Nintendo is seriously pushing the balance board. In the US WiiFit is one of Nintendo's biggest ad campaigns ever (maybe Wii launch budget topped it, ) and they've nudged a few developers to follow up on it. With WiiFit's install base, 3rd party developers may have a reason to develop for it anyway.
In general, most peripherals don't sell well, or sell to an exclusive audience then die off. Anyone who remebers Arcade Sticks, 6-button controllers, that Capcom Mech game (*shudder*), or multitaps have a good firsthand memory of this.
There is no such thing as a console war. This is the first step to game design.
