Not accounting for tastes and preferences, but it is decidedly not true that tmux does everything Zellij does (just some very minor examples are: true floating panes, true multiplayer support, stacked panes, etc.)

As for prevalence: this very often has to do with the distributions themselves and their ability to accept certain (rust) dependencies.

Otherwise though, I always say Zellij is not competing with tmux. That tool choices should be more equivalent to clothing choices than to choosing a school for your kids. Choose what you like, there should not be religious disagreements here.