I think it's just because it's a relatively rare word (https://books.google.com/ngrams/graph?content=nonplussed%2Cbewildered%2Cconfused&year_start=1800&year_end

I think it's just because it's a relatively rare word (https://books.google.com/ngrams/graph?content=nonplussed%2Cbewildered%2Cconfused&year_start=1800&year_end=2019&corpus=en-2019&smoothing=3 https://books.google.com/ngrams/graph?content=nonplussed%2Cb...), and it's often extremely difficult to figure out what it means from context ("He looked nonplussed"), so people just try to guess based on the word itself. Plus means like more of something, so not more, not a lot, something like that. "He looked not a lot", I guess not very interested. Plus might mean more of a reaction, so nonplussed is no reaction. Simple as that. I don't think most people paid as much attention to their high school reading as you're imagining. Also note that two of Merriam's examples are from before 1984 came out.