The thousands of canceled flights are over millions, and the average person doesn't fly that much. Around one round trip a year in the US.
In fact, while I am very aware of the B737-MAX fiasco because I am interested in the technical side of things, I didn't even realize there were thousands of canceled flights. Sure, it is something I should know, but it is not like canceled flights are a remarkable event, and because I wasn't affected by it, I didn't really think about it.
How many people do you know who frequently took flights right at the time when this was all happening? If I were not an avid reader of the news, I'd have no clue. Before this, I also would not have cared to check what airplane my flight was when I did fly.
Are they canceling those flights, or are they not offering them? It's a big difference. People notice when a flight they've already booked is canceled. They don't notice flights that were never shown to them in the first place. I fly regularly between Boston and Seattle. I've noticed that my choice of flights is less than it was, but if I were a less regular flier on that route I'd have no idea that the selection was even reduced let alone of the reasons.
My circle of friends is a pretty wide net and only one coworker knows about the Boeing nonsense, and he admitted it was because of Hacker News.