Why doesn't Quartet use USB 3.0?

The USB 2 spec's bandwidth is more than adequate for the channel count of the Quartet.

The USB 3 spec doesn't get the data to its destination any faster.

This is often confusing as you will see lots of marketing about USB 3 being faster, however this is meant in situations like transferring data between hard drives, packeted data, not real time audio streams.

A good example would be cars on a highway.  The speed limit is 65mph, so making the road wider with more lanes would mean more cars could travel on the road, but each individual car will still travel the distance in the same time.  This is also true with USB 3 vs USB 2.

On USB 3 connections a hard drive can push X amount of raw data in a shorter amount of time, as there are more "lanes" for different packets to travel on.  This is why you will see that USB 3 hard drives are rated as "faster" in that they transfer a specific amount of data in a faster time.

But with real time audio streams, each stream will still have the same round trip time on USB 2 or 3.  So USB 3 allows for more channels simultaneously, not lower latency of each channel.