I think you’re correct, it’s just that the people who are working on immutable distros—like Fedora—have strict policies about open-source software, and snap as a proprietary distribution method can never be officially supported as a result. To be fair, your question about why distros don’t support the /snap folder/filesystem is just as valid as wondering why snap doesn’t change itself to support immutable distros like Silverblue. The work could be put in on either end and neither one seems to want to do it, so here we are.
It’s obviously possible to use snap on an immutable OS because Ubuntu Core exists. When Canonical finally releases a desktop version of Ubuntu Core, they’re actually going to do the opposite and not support Flatpaks, so there’s a political component to this too for whatever reason.