Some general things: Both Tmux and Neovim have such broad plugin ecosystems, helix and zellij don’t even come close.
For zellij specifically: I don’t like the UI, it’s just way too much stuff on the screen, it’s distracting. Tmux tries to stay out of the way and only displays something if it’s absolutely necessary. You can also configure everything yourself.
Regarding helix: As a long time Vim/Emacs-evil user I just can’t get used to stuff like Kakoune or Helix, it just feels weird.