Because currently you need support of WoL from both your NIC (to enter low power WoL monitoring mode) and your motherboard (to listen for WoL events from the NI

Because currently you need support of WoL from both your NIC (to enter low power WoL monitoring mode) and your motherboard (to listen for WoL events from the NIC and trigger power on). If the motherboard would embed a simplistic NIC/WoL driver then you could have WoL on any NIC, with the motherboard acting like any client. I suppose that would mean using the PCI lane and stuff to communicate with the NIC though which may defeat the purpose of a dedicated low power interface.