Most of the core community moved to OOC when the admins changed. A number of people stayed on HP because they didn't like how OOC looked, but I suspect the majority of people stayed simply did so because they don't use the forums enough to even know what was happening between the two sites at all. Generally speaking, people who don't use forums often are more likely to lack forums etiquette and thus make shitposts and other less than constructive posts more often
It might also be a matter of passion. People who are passionate and positive about the game are more likely to make constructive and positive posts. Sharing good posts and posting often makes you more likely to bond with the community. If you just make shitposts and salt posts, the community will react negatively, you will get a negative experience and you obviously won't bond with the people shaming you (you might even hate them). Anyway, people who were passionate about the community and the admins who held it together were more likely to migrate to OOC
I don't get how this is anything other than opinion based but best of luck to OOC and the people there. The community here at HP is unrivaled and that hasn't changed far as I'm concerned.