Sequels were made and watched in a very different way back then. Everyone has a raging boner for continuity and tonal consistency now, but it just didn't matter as much to the average, non-starlog reader. That's why it was ok for James Bond to meet Blofeld in one movie and then meet him again in the very next. It just wasn't a big deal.

I think the shift in sequel-making mentality might have begun after Temple of Doom was so deliberately, gleefully, different in tone than Raiders and people got all pissed.