Quite simply, USB 3.0 doesn’t provide any benefit for digital audio over USB – USB 2’s bandwidth is more than adequate, and USB 3 doesn’t get the data to its destination any faster.
Imagine an airline route between two cities – planes take off at regular intervals and take a certain amount of time to arrive at their destination. If the airline were to double the size of their planes, the number of passengers that can fly between the two cities – the bandwidth – doubles. But each passenger doesn’t get to their destination any faster. Moreover, if the smaller planes can easily accommodate the number of people who desire to fly that route, there’s no reason to run the larger planes.
Likewise, the bit-rate of a digital audio signal, even with several channels at 192kHz sampling rate, is easily transferred over USB 2. Digital audio doesn’t arrive at its destination any faster over USB 3, thus latency doesn’t improve.