Because many fans will not move over, that platform is not nearly as big and able to work with the same payment providers. Now that might change if all of us fans moved over and stopped supporting on Patreon as we likely should. For now though Patreon has still got close to a monopoly and can do whatever it wants in practice as shown in your post above mine. Still the censorship is by Patreon not Brain, so if have a problem with it go complain to the censor, not the dev that is forced to comply or risks to loose a big chunk of his income on a point of principle. Always easy to tell others to risk their income instead of acting yourself.
Edit: For myself I was just to late this month, but due to the treatment of another game that was well within the rules even if had some incest I decided to close all my Patreon supports before November and will try to support those devs I supported in another way if available.