Starter motors are built really robust and the chances of them failing are very slim. So, the reasons can be one or more of the following as far as I know.
1. Repeated and prolonged cranking - will eat away the windings like a kid eats chocolates.
2. Short circuits in the leads which might have burnt out the coil endings.
3. De-enamelling caused by excessive heat which might have caused a winding short - instant motor death.
As a rule of the thumb, it's best to follow these steps:
1. Never crank for more than three to five seconds.
2. If the car does not start in the first attempt, give 30 seconds to cool down the windings before re-cranking.
3. Keep the engine at the least possible load during starting - AC off, Stereo off, lights off etc.
4. Periodically inspect the engine bay for any signs of abnormalities in the wiring - loose connections, fissures in the wire sheaths, oxide deposits etc. Always a good practice to open the hood once in a while