Because, when you post here from kbin.social any other instance with kbinMeta@kbin.social will get a copy of that and vice-versa. But each side is exchanging posts from multiple magazines to multiple other instances. It's also balancing resource usage for people visiting the site too.
Also new instances are gradually fetching the back-catalog of posts for various magazines (and communities on lemmy). So all of this leads to a delay.
Anecdotally the delay is quite short this morning. Yesterday it reached up to 2 hours from my view at least.