We’ve also been looking for these for a long time, but no one seemed to make a USB 3.0 Gigabit Ethernet adapter until just this year (2012).
But we needed them for a different purpose; we don’t do virtualization but needed to add second full Gigabit Ethernet port to the laptops we send out to be used as network diagnostics equipment. We used to use ExpressCard before, but laptops don’t have this slot anymore, so had to get the USB 3.0 to Gigabit Ethernet adapter. All others wouldn’t actually run at full Gigabit Ethernet speed despite showing it as being Gigabit Ethernet capable. The throughput of USB 2.0 was too low.
As to why it took so long, the reason is very simple actually: Nobody thought this was a product worth making until Axis decided to do it. Once they made the chip, PCB layout and programming of the chip then at least 3 companies began making the Gigabit Ethernet adapters. All of them are based on the same ASIX AX88179 chip, but the one on the site above is the cheapest we could find.