"Once the film came out and was this huge success, the studio very quickly said, "We must have a sequel!" But because the second one was such a long script, and they couldn't find any way to shorten it, they decided to make it as two films. I'm surprised the second one wasn't marketed as "part two of three". A lot of people found it less rewarding because it didn't have the fun and lightness of the first one, but then the third one makes sense of the second – it all connects up. I think the first film resonates so well with audiences because of the element of wish-fulfilment at its heart. We've all said to ourselves, "I wish I could go back in time and change something" or, "If only I could do that over again"."
January 1, 1970
https://en.wikiquote.org/wiki/Back_to_the_Future_Part_II