Now I'm not an insider, but I would imagine FPC has quite a few advantages over Patreon, including:
Being/becoming a video platform with much higher quality streams and download
Being their own platform, thus them not being dependent on another organization
And there are likely many more very obvious benefits to their own FPC, compared to Patreon.
Plus I think in the past they have said in the past they don't taking people's money without giving anything back, which they haven't given people much back with Patreon I think.
Because they want to develop floatplane as a brand new platform many creators can use, like Netflix except for channels, instead of piggybacking on other parties.