The purpose of developing a game is to generate a profit. The profit that you can make depends on the potential for the game to sell -- for example, if your game type is very popular on the 360 and only sort of popular on the Wii, then you may have more potential revenue on the 360 even through the Wii has a greater user base, assuming approximately the same saturation level for that genre.
Profit is also tempered by costs. Your revenue needs to exceed your costs if you realistically hope to generate any sort of profit. Different companies will have different formulas for each system, but ultimately the PS3 tends to be the most costly system because of difficult in development. The 360 is far closer to traditional development so you need less console specialization -- this means you have your pick of more developers and generally less expensive ones.
So 360 costs tend to be lower than PS3 costs. This means developers will move to 360 first until a given genre is overly saturated -- for example, First Person Shooters could be saturated in the 360 or there could be too much high end competition for FPS games on the 360, so you might release yours on another system.
