If you think in terms of voltage on a pin, how you would implement ternary? Binary is simple: 0V, or 5V (at least a long time ago...). Transitioning from one state to the other is obvious. How would you do it with ternary logic?
Do negative voltages exist?
Positive voltage, negative voltage, no voltage
Ternary allows for measurements of relativity at the pin level whereas binary is a yes/no...
:-)