> Why exactly do you think that is?
Canonical very publicly collaborated with these companies to get their software on the Snap store while Flathub has been handled in a more free-for-all manner. If you build it, they will come, and all that. Moreover, I'm not so sure Red Hat is as explicitly interested in monopolizing the Linux application ecosystem as Canonical, especially since their acquisition by IBM. Maybe IBM has other ideas, but we likely won't see similar marketing efforts on that front in the near future.
The microsoft, spotify and Jetbrains software comes directly from their respective publishers. Problems are supported and debugged directly. That is a major difference.