in addition to the concept of groups, posts and comments you now have instances, threads, microblogs and magazines. That's overwhelming and drives people off. The masses don't care how it is technically implemented as long as it's easy to handle.
The only new concepts on /kbin, compared to reddit are: instances and microblogs.
Multiple instances (servers) are the essence of fediverse. Microblogs exist only on /kbin, because Mastodon (as it is much bigger than Lemmy right now).
Groups are subreddits are communities (Lemmy) are magazines (/kbin).
Threads is your regular reddit-style posts and comments. Lemmy is nothing but threads. Same for reddit, innit?