USB isn't the right choice for a lot of interconnects, or even most interconnects, especially things like HID for mobile devices. One of the big advantages you can have with I2C is that because it's multi-master, both sides of the HID chain can become a lot simpler. The host side doesn't need to send occasional messages asking if the various HID devices have data, and the device side doesn't need the more complicated logic to handle USB HID events. Instead, the HID device can send HID events as they happen, and the host only needs to send the much less common host communications -- for commands like turning on and off lights, etc -- when those happens. This means a bit deeper sleep, and as such, less power consumption.
