In my case how "isolated" my country was before internet got popular in mid-2000s is the reason for why we had no idea it was called "anime" and we wouldn't notice it's something Japanese if the animes didn't have Japanese text. So many technologies and way of living always came late here until around 2010s. For example here playing video games was a rare thing until mid-2000s despite arcade was popular but its popularity was a flame quickly extinguished in half a decade because "it's stupid to keep playing for it" and it's why Atari 2600 -> fake NES consoles and Sega Genesis was so popular but only among niche gamers when not many people played games and knew they exist. I was living in a big city and there was only 1 shop selling video games and whatnot for a long time and it became 2 and 3rd and 4th was too late so they are shot down by new "anti-piracy laws" lol.
As for why lots of people grew up and stopped watching animes, it was because "it is a cartoon and therefore it's childish" thing. I also thought so too because Dragon Ball and Pokemon was childish AF. Captain Tsubasa was slice of life yet why it was childish was because whatever happens is against laws of physics lol. As you get old you get more sensitive to "what doesn't make sense" unless you are person who knows how to enjoy life or you go crazy. So I didn't take animes seriously until I met one of my best friends who is too into animes. He taught me these are called animes and then he introduced me Death Note. I was interested in it because it wasn't childish and no "stupid anime shit" in it, and then I started to give chance to anime and thus I really found legendary animes like Psycho-Pass and all so I realized there are 4 aspects of animes: Animes for kids, animes for girls, animes for perverts and then animes for grown up people (I don't mean porn or some shit, I mean animes that actually have a sign of intelligence it aims for instead of playing for primitive urges). Then I met people who watch animes but they only limit themselves to animes for grown up people so they also introduced me to some animes. Without them I guess I would always think animes are for little kids or pervert insane people lol, however not many people are open minded as much as me and they won't have people in this country who is into anime. Because I'm open minded that's why in my country I was present in very different social aspect of my country enough to meet tons of people in my life to gain POV from every side to get to know about my society. And because I'm trying to be objective and being open minded as much as possible and all I care about is fun that is why I was a person who could change my mind about animes but as people get old they tend to be narrow minded and get lost in the jungle of their "should" and bias statements. Which is why for anyone whom anime didn't mean anything to them personality when they grow up and anyone who never introduced an aspect of anime that their grown up adult ass can relate they will never stop underestimating animes after they will leave it behind to feel "adult" just like stopping playing with toys and kill the child in themselves that actually kills their humanity. What a shame!!! lol