According to the devs involved, on mailing lists and bug reports, the point of snap over apt/etc is the auto updates can't be disabled, so end-users can't put off or forget about security updates. Even adding a way to delay or configure when an update happens seemed to take a lot of convincing before it was added.
(In the end I just disabled the snap service entirely to stop auto-updates. Only downside seems to be that I can't query or install new things through snap without it.)
Snaps are not Ubuntu-only. You can find install instructions for many distros here: https://docs.snapcraft.io/core/install
On the other hand,flatpak is a freedesktop project, done using open standards like OCI and ostree.
There is room in the world for flatpak and snap to co-exist. We created snaps as an evolution on from clicks on the Ubuntu phone, and it covers use cases that flatpak wasn't designed for.