Reddit kind of got lucky with the development of modern smartphones.
Old format forums that were designed for desktops were way too cluttered for mobile, especially with how small screens were back then. Reddit comes along with its streamlined take on forums as well as the ability to have a forum for any and every subject all on one site and it just took off.
Reddit got to where it is by relying on the labor of others. The original site code was open source, the mobile apps were made by other people, users moderated the subs for free, and users generated almost all of the content.
Reddit began as a clunky forum and was popular long before smartphones, though. And like the other person said, they didn't have an app for a long time. So this take of yours is a little flawed.
It wasn’t popular long before smartphones. It was known about, sure. But the development of modern smartphones is what made Reddit one of the biggest sites in the world.