Before revision: Because there is no p-polarized reflection (oscillating along the plane formed from the axis of light travel and the vertical normal of the surface) from the pure liquid when both are angled towards it at the Brewster angle, light is only reflected when some other phenomenon such as a surface film affects the liquid surface.
After revision: Because there is no p-polarized reflection from the pure liquid when both are angled towards it at the Brewster angle, light is only reflected when some other phenomenon such as a surface film affects the liquid surface.