I think it's another fine example of Canonical pushing its own products rather than supporting and enhancing existing standards (flatpak and appimage), which people are getting tired of. Also, as I understand it, the snap store itself is proprietary and is therefore controlled by Canonical.

The server isn't open source, so Canonical has the sole ability to control snap distribution. It's also yet another example of Canonical's "Not Invented Here" syndrome, where they constantly reinvent things so they can control it instead of working with the rest of the open source community. They also trick you into using snaps; for example if you explicitly tell it to use apt to install Firefox, it'll install it as a snap anyways.

Historically they performed really poorly as well, but my understanding is that they've largely fixed that issue.