Because instances getting larger are going to incur exponentially larger costs. If the largest communities start suffering from performance issues or something like that, it fragments the community into who knows how many instances the community will be split into. The only problem Lemmy has is lack of users and engagement, and I think it’s actually flawed by design in this way. It’s like having a big party, but instead of everyone in one house you split it into a bunch of them with not that many people or food options. Idk if that’s ideal or not.