Gold is one of the most malleable metal we have and we could extract thin foils from it but other metals are not that malleable. So if we use a metal other than gold in the alpha-particle scattering experiment we would not being able to get a thin foil. A comparatively thick foil means that the number of atoms per unit volume would be large and hence the number of alpha particles passing through the foil without being deflected would decrease and those getting deflected would increase in comparison with the gold foil.