I think it is just a ton of loving attention to detail in the sets, costumes, music and performances. Crucially for movies of that era, I think the CGI is also

I think it is just a ton of loving attention to detail in the sets, costumes, music and performances. Crucially for movies of that era, I think the CGI is also tastefully done and blended with the rest of the film. They didn’t go for much of the kinds of full CG scenes we see in todays Marvel movies you could make an apples to apples comparison for and see the gulf in techniques you get over a few decades, but they also didn’t use techniques like stop motion or muppets that seem aged to us when looking at 80’s films.