Why only gold leaves are used in electroscope?

Because they are malleable.

Gold is not the only metal used in electroscope. Copper is also used in electroscope.

• A reason why these metals are used is that they are malleable more than any other metal. To use the electroscope, you need to put a thin trey into it.

• Another reason, which is not the best and is not quiet sure to me, is that Gold is less reactive. If you put a highly reactive metal as trey's substance, then that metal will react with the electrons that pass through it and will leave the observer a hard chance of having a good view.