If you are encountering snap not found error for some Snap packages, this tutorial will help you find why.
I'd say because snap is shit. It is really sad to see that this will become Ubuntu's standard.
Why shit? It's good to have simpler packages to install. Right now, if it's not in the repositories, you can only compile packages manually (which is no doubt a huge wall for users).
Not entirely true. See this old article on Guix which also compares to Snap a bit (iirc) http://www.ocsmag.com/2016/07/05/guix-the-non-aligned-universal-package-manager/ .. another option is Nix. Both Guix and Nix can run on (most) existing systems.
First you have to ask yourself, why is it not in the repositories? Who will maintain the dependency updates (especially reagrding security) of your snaps?