What prevents joule thiefs from operating at input voltages less than 0.3 volts? Is it the transistor?

I know that joule thiefs can operate at input voltages of 0.3 volts, but why cant they operate at input voltages of less than 0.3 volts? Is the joule thiefs capability of utilizing lower amounts of energy limited by the transistor? i.e. the transistor cannot operate at voltages of less than 0.3 volts? Or is it something else? Please be very specific, im trying to understand why this is. Thanks!!