The problem is engagement - which in turn means building a community.

If someone goes to your forum, hijacks your posts, and them puts them on Reddit; there is no incentive to use your platform, which stifles and dies since no one is communicating on it. This is also part of the tyranny, as people can figuratively walk into your house, eat all your food, and then expect you to restock.

On the old internet, this was not really an issue since there was a better culture. But if all your friends use Reddit, and all the content is reposted there, why use the internet forum?