The 8 bits are limited by the clock frequency necessary for the PWM generation by the micro-controller. With an 8-bit PWM a clock of 10~12KHz is required which drastically reduces power consumption and is helpful in hazardous environments. With 16-bit PWM your clock rate will need to be much higher which leads to higher op amp bandwidth. Another limitation is the 390ns minimum pulse width for the PWM to maintain 4-20mA accuracy.