We're not having a hard time getting back there; we're just not trying. And with the "wind" argument- space is a vacuum. Therefore, there is no friction (wind resistance) preventing the flag from swaying the flag back and forth. The simple act of placing the flag on the moon would cause vibrations to resonate up the flag pole (not to mention the act of carrying the flag). The tension in the flag would cause an effect of elasticity and simple harmonic motion in the banner.