I don’t know which decades you are referring to about the squishy-minded thinking the Mayans were such nice guys. When I first became interested in Mayans about 55 years ago, I was struck by their human sacrifices throwing people into the sacred Cenote wells.
http://www.utexas.edu/utpress/excerpts/exevarom.html
It started from the time of their rediscovery in the 19th Century. At the time, America was full of wacky ideas, so when John Lloyd Stephens and Frederick Catherwood published lavishly illustrated and inexpensive books about the Maya, every fantasist latched on to it for their own agenda.
Ever since Jean-Jacques Rousseau had idealized his Garden of Eden parody, the State of Nature, there had been a popular search to find primitive peoples and cultures who lived that fantasy life. And with their elaborate cities and art, it was easy to imagine the Mayans as living in a pre-industrial paradise.
Only when professional archaeologists began examining their rather loathsome culture, did the light finally start to dawn. But the inertia of what many wanted the Maya to be, and what they actually were, lasted for a long time. Even today, with dumbasses convinced that the Maya could see into the future a thousand years after their own civilization crumbled.