Apple is driving the MacBook’s USB data capabilities straight off of the Core-M processor, which in turn only supports USB 3.1 Gen1 data (5Gbps Superspeed), which is otherwise equivalent to USB 3.0. To support Gen2 speeds (10Gbps Superspeed+), Apple would need to use a 3rd party USB controller at this time, which are still very new and would compromise the size of the logic board, along with potentially compromising battery life.