Say what you will about reddit, at least an established subreddit was the place to gather on the topic, ie r/technology etc.
This premise on which your question is based isn't actually true though. There's /r/technology and also /r/tech. There's /r/DnD and also /r/dndnext. As of recently, for some reason there are like 35 nearly identical amitheasshole subreddits with different names.
I feel like what you're observing is just that reddit communities are mature, people have had time to gravitate to whichever community is more active or has better quality moderation and so there is generally a "winner" sub with more participation because... unless there's a major problem with the bigger sub it tends to be more interesting than a less well-trafficked sub.
Lemmy, in contrast, is still fairly wild-west. Most communities are not very active and have only a few subscribers. If a competing community with an overlapping topic appears, folks are willing to subscribe to it just in case it takes off. If Lemmy continues to retain a healthy number of users, I expect in most cases that consolidation would set in unless there were major differences in moderation policy or something else that splits the community into factions that align across server or community boundaries... and over time you'll see a similar layout of one or two dominant communities and a long tail of tiny ones that few pay attention to.