Why are NPN Darlington transistors used to sink current?
Sinking load switches with an NPN darlington allow the control signal to be a GND referenced signal. If you use high side sourcing switches it is most typical that then the control signal needs translation down to a GND referenced signal domain. 

These days when MCUs are controlling almost everything the GPIO pins on such devices are GND referenced signals. And so it should be obvious why many load switches use the syncing type components with a GND referenced input.