Because snaps are terrible. They constantly break parts of apps for no reason. If you have container issues with a flatpak, just use flatseal to punch a hole through the container. With snaps, people will tell you to install the non-snap version because that's easier than beating snap into submission. I learned that the hard way when I had a university project with kubernetes and docker was installed as a snap. I spent way too much time trying to make it work at all before giving up and switching to a VM on my work laptop where it went surprisingly smooth without snaps.

Flatpaks are better in every way and since this isn't about money, we should all just move on and use the best tool for the job.

But what does canonical think should happen when you run sudo apt install firefox and press Y? That's right, you now have firefox as a snap. Have fun waiting for 5 seconds every time you start it.

Shit like that scares new users away from linux as a whole.