Thunderbolt is clearly the future for connecting the highest bandwidth peripherals to your Mac. For the peripherals that require such bandwidth, the higher cost of Thunderbolt components and accessories (such as cables) is worth it. High resolution video, for example, requires significant bandwidth so its associated peripherals would undoubtedly benefit from Thunderbolt connectivity.
Quartet's bandwidth requirements are comparatively small next to high-resolution video peripherals, and are easily fulfilled by USB 2.0. The latency with USB 2.0 Audio is excellent - just 3.6ms (roundtrip) when recording in Logic at 96KHz/32-buffer. Plus the inclusion of Thunderbolt would increase Quartet's cost significantly without adding a great deal of benefit.
Finally, being a USB 2.0 unit allows Quartet to be compatible with iPads and iPhones, a great benefit over Thunderbolt devices, which require a full size Mac.
In the end, USB 2.0 is simply the most appropriate connection for Quartet, providing more than enough bandwidth and low latency at a reasonable cost, and wider compatibility.