I read so many books as a child, and I read so fast o.O But I usually didn't remember the plot at all, I didn't really understand that remembering the plot was a thing people did, lol. (Until I met people who actually remembered the plot of Harry Potter and were smug about that.)
I remember being a teenager monologuing to a friend about being too shit to deserve books, and hating books for that, or something, so I guess the answer to "why did I stop reading" is depression, just like the answer to most other questions ✨