The maximum allowed round-trip delay is the primary reason for these limitations. If USB host commands don't get any answer from USB device within the allowed time, the host treats the command as lost. In order to meet the time standard the length of cable should be limited.
The limitations while using USB cable
Each USB cable can't be longer than 5 meters16 feet (5 m) is the farthest USB signal can go without causing delays. Of course it is impossible to bypass this limitation by joining several cables in one long and connect a USB device so far from your computer.
Each USB device can't have more than 5 "nodes" between it and the PCHub, switch, or repeater is a "node" that allows more devices to be connected to USB network. There is no possibility to connect a scanner via 6 hubs as the result. The space in the addressing scheme is not enough for the PC to send or receive data from scanner.
Not all virtual machines can work with USB devicesUSB devices are frequently needed in virtual sessions, but still they cannot be connected to virtual machines directly.
The Universal Serial Bus allows connecting up to 127 devices to a computer.