Relaxed regulations back then will be a factor, both externally enforced and internal rules designed to stop past mistakes repeating.
Another significant consideration is the growing complexity of each individual installation. Also the higher expectations and attention to detail of the modern customer: you don't have the luxury of hiding a few flaws behind the spectacle of novelty, now such theme parks are no longer that novel.
I think what you're describing is a great example, checkout one of the classic rides, It's a Small World:
https://www.youtube.com/watch?v=7tbm4Au3dZ0
vs the new Smuggler's Run
https://www.youtube.com/watch?v=3IZDfFDbGnU
An astounding leap forward in environmental storytelling, attention to detail, technology, etc.
Might be relaxed laws but not because we were cooler back then,more likely because we didn't know yet that some practices were dangerous.
It's also pretty remarkable that they finished the entire thing in a single year. For a modern comparison the new Star Wars expansion took 3 years to complete with just one working ride (the second ride will be open in December, so 3.5 years from construction start). I wonder if that's due to more relaxed laws surrounding construction back then or another factor.
[1] https://mentalfloss.com/article/541360/disneyland-disastrous...