Cost. USB is a very complex protocol that requires a controller. For most embedded systems a serial port, which is very very cheap and dead simple to implemen

Cost. USB is a very complex protocol that requires a controller. For most embedded systems a serial port, which is very very cheap and dead simple to implement, is sufficient. There's really no need for a USB port on the Roomba, so it's silly to incur additional costs on every single unit to add it.