Cause Snap is proprietary and the repo is controlled only by Canonical/Ubuntu.
Flatpack on the other hand allows for third parry repos. And then one might even preference a classical install because ironically its snappier - faster - especially on oldee setups
The snapcraft webserver backend is closed source but everything snap adjacent that touches your computer is open source, and you can distribute snaps and install them without using the snap store
They are 1. Slow on launch and on boot. 2. Very opaque and offer little control. 3. They don't integrate well. 4. Proprietary. 5. Probably most importantly, Canonical has made 0 progress and demonstrated 0 interest in improving issues 1-4.