Yes, a faulty blower motor resistor is a very common cause of a car's blower motor only working on high speed. The resistor is responsible for reducing the voltage to the motor, allowing for different speeds. If it's failing, only the highest speed (which often bypasses the resistor) will function.
Since you've already replaced the control motor (which is a different component), the blower motor resistor is a strong suspect. It's relatively inexpensive and easy to replace, so it's worth checking and/or replacing.
