Nintendo has a more tight quality control when it come to finished products. MS has a HUGE history of releasing rushed products and then fix them later. I am not sure about the PS3 but I have heard that most of the first and second generation PS2 had drive failures issues.
Other thing may be how the consoles are designed. Because Wii doesn't have dozens of chips and a hard drive, it consummes less enegrgy and doesn't heat a lot and that keeps the hardware safer.