In all the research there is no number one reason. We also could use more symptoms of the problem. But, the general rule of combustion is, if it will not start it is either fuel or fire. If you are getting fuel and the engine is spinning then there is an electrical problem that most likely can only be diagnosed with a computer.  

If it is firing on all eight cylinders then it needs to get fuel. This can be checked at the fuel rail to see if it is getting enough fuel to push through the fuel injectors. Most MPI systems will need 40 to 90 pounds of pressure depending on the vehicle.  

This is all assuming that the engine will spin over.  If not a battery, starter and/or a solenoid may be bad.

Gerald Wheelus
Edgewood Auto Parts #253
Edgewood, Texas