I've seen lots of articles about Snappy and didn't even know that Flatpak existed.  Granted I don't follow Gnome development and am more interested in KDE and LxQT - but that said, I'm not particularly interested in Ubuntu either.  If the idea behind flatpak is to make more packages available, it ain't going to work if people don't know about it.  Most people will just choose snappy or flatpak, ,and since both work - just use the snappy format.  It's like Beta and VHS or more recently HD DVD and Blu-ray.  If you have a universal format, one will become dominant - and for better or worse, it's not necessarily about which one is better, it has to do with marketing.