Why is voltage also called potential difference?

Your question should really be the other way around! Technically, the correct term is 'potential difference'. Since this is measured in volts, over time it has also become known as 'voltage'. A similar thing has happened, but to a lesser extent, with 'power' which, being measured in watts, is often referred to as 'wattage'.

So, 'voltage' is simply another Word for 'potential difference'. Originally, it meant 'potential difference expressed in volts' but, these days, it applies to microvolts, millivolts, volts, kilovolts, or megavolts!

Take care, however, not to confuse 'voltage' and 'potential'. These are different, and voltage should never be used when you mean potential.