Reddit and Discord are very different. Reddit is a holdover from the '00s phpBB /somethingawful /slashdot /digg messageboard era. Like messageboards, it was always public, which is how people discovered it, and went on to join specific communities.

Discord is a instant messaging product that reflects modern social media priorities of walling everything off behind a user login; anathema to the public-facing nature of Reddit and Digg. If there's anything that prevents regular people from joining Discord, it's that you actually have to be told to join a specific discord, for something you're interested in. You can't click a link from someplace and see something interesting happening there.