It was part of a 20th Century retrospective, and Star Wars was selected to represent the 1970's. Lucas denied them permission to show an original print, thus missing the whole point of the festival. The '97 was shown in it's place.

Lucas has prevented several other screenings since then. I'm surprised he hasn't tried to take back the prints the Library of Congress and the British Film institute have in their collections.

There were limited showings of the trilogy timed to when the AFI sucked up to give Lucas an award in 2005. They were the '97 versions again.