I’ve seen a few reasons.

New communities are created, a very specific and relevant post is made(which is likely the reason for making the new com), and it rides naturally through new/hot/all.

New communities are made as a direct discussion in a com or instance so people already know where to go as soon as its made.

And then some are made as alternatives to others due to conflicts so there’s a side that stays and a side that leaves to the new com immediately.