 When "Hill Valley" was created for the original movie they built the  town in the pristine 1955 condition and shot the middle of the  movie, then damaged it for the 1985 town and shot the beginning and  end of the movie. When they decided to shoot BTTF 2 they had to  clean the set up and restore it to the same condition it was in  1955. It cost more to rebuild than it cost to build it from scratch.