Because the physical circuit design of USB 2.0 and USB 3.0 is different.​
A USB 3.0 connector includes the four traditional USB 2.0 lines (VBUS, D+, D–, and GND),​
plus an additional five SuperSpeed signal lines (TX/RX pairs) dedicated for high-speed data transfer.​
However, a USB 2.0 device only uses the D+ and D– differential pair and does not connect to the SuperSpeed lines.​
Therefore, when the xHCI controller enumerates the device, it detects that the device supports only the USB 2.0 mode.​
As a result, the port operates in High-Speed (480 Mbps) mode and does not enable the SuperSpeed (5 Gbps / 10 Gbps) .​