It's rooted in racism. Chinese American food was cheap and delicious, steadily growing in popularity. Non-Chinese restaurant owners viewed us as a threat. So the racists among them used dubious studies, which have since been discredited, to try to spread this myth that Chinese food was unhealthy because it contained MSG.

Of course it was all a fabrication. People would claim to feel bloated/sick even after eating Chinese food containing no MSG, probably because it tastes so damn good and people are bad at eating in moderation. Likewise, people could eat non-Chinese food containing MSG and somehow there are minimal complaints.

My family's restaurant was put through the ringer over this in the 80's-90's and almost went out of business, but thankfully the science prevailed in the end.