It boils down to basic shielding and radio interference issues since USB 3.0 creates “noise” in the 2.4–2.5GHz spectrum.
From a white paper titled “USB 3.0* Radio Frequency Interference on 2.4 GHz Devices” written by Intel on the subject:
As previously shown in Figure 2-2, the noise from USB 3.0 data spectrum can be high (in the 2.4–2.5 GHz range). This noise can radiate from the USB 3.0 connector on a PC platform, the USB 3.0 connector on the peripheral device or the USB 3.0 cable. If the antenna of a wireless device operating in this band is placed close to any of the above USB 3.0 radiation channels, it can pick up the broadband noise. The broadband noise emitted from a USB 3.0 device can affect the SNR and limit the sensitivity of any wireless receiver whose antenna is physically located close to the USB 3.0 device. This may result in a drop in throughput on the wireless link.
It basically all comes down to shielding. If the cables and connectors are not properly shielded and a 2.4GHz device is placed nearby, the USB 3.0 signaling can leak and—as a result—interfere with the device.
Here is an illustration from that white paper titled, “Figure 3-1. Radiation from USB 3.0* Channels”; the caption reads, “As an example, an external USB 3.0 hard disk drive (HDD) was used to assess the level of noise that can be radiated from a USB 3.0 device.”