In part, because the higher speed of the USB 3.0 port (also known as USB 3.1 Gen 1 and USB 3.2 Gen 1) offers no advantage if the device itself is USB 2.0. For Plugable's USB 2.0 Bluetooth and Wi-Fi adapters, the performance is far below even the throughput of USB 2.0.
Also, the adapter itself is a USB 2.0 device and there is a critical issue that was overlooked in the design of USB 3.0 specifications. Many USB 3.0 ports leak broad-spectrum radio interference in the 2.4GHz to 2.5GHz range, which is the same radio band used by Bluetooth and Wi-Fi
While this interference can be reduced through heavy shielding, it cannot be entirely avoided as it is inherent to the core design of USB 3.0. USB 2.0 ports do not have this issue. However, a USB 2.0 port that is in close proximity to an in-use USB 3.0 port may also be affected by radio noise leaked by it.