Small DC fans, what decides their working voltage?

[kalel]

In small DC fans, like computer fans, working voltage is almost always 12v, this is obviously designed for a reason.

Let's disregard PWM-supported fans as those have fancier built in controls.

What determines the working voltage of such a fan? The number of loops on the coils? Added resistors on the circuit boards?

[jeroen79]

The working voltage would be determined by how the motor is built (number of turns in the coils, wire thickness, ventilation, max rpm of the bearings, etc) and Ohm's law.

Higher voltage is more current and more speed so more power and more heat.
And eventually something will break.

In a meeting modern PC the 12V rail would be the best choice to power the fans so they will be designed with that specification.

[Gregg]

The higher the voltage to a motor, the smaller the wire needed and since most modern motherboards control the fans, it means smaller traces can be used.  Also as conductors get hotter they have greater resistance meaning more voltage drop and the higher the voltage, the less the fan is affected by voltage drop. 
An internet search for computer power supply rails will lead you to the history of how and why modern computers have most of their power available at 12 volts.