Funny - I asked that question three or maybe four years ago to manufacturers who were introducing Thunderbolt interfaces. The answer was always along the "chicken-and-egg" line. There weren't enough Intel motherboards with a Thunderbird port available yet for them to justify work on a driver, and that they'd support Windows when Windows supported Thunderbolt. And the motherboard manufacturers said their customers where happy with USB and weren't asking for Thunderbolt.
I guess it's still that way. Maybe somebody should remind UA and MOTU and Focusrite (and Intel, Asus, MSI, and Gigabyte too) that you can plug Thunderbolt devices into a USB-C connector and those are starting to show up on Windows motherboards.