I think a lot of it is that tmux had friendler, sane defaults and clearer command design (eg. status bar out of the box). And, I suspect, people not already knowing screen.
The latter reason is why Helix is slow in doing similar to vim, I think, despite being far more consistently designed and saner defaults. Everyone knows vim exists.
I've still never switched from screen, personally, though I'm only a light user.
Defaults matter. I've had my screenrc with a status line but I still ended up preferring tmux in the end, because not needing a config file was a small but pleasant advantage when working with different machines. I also didn't like ctrl+a as the leader combination, because it conflicts with "go to the beginning of the line" in bash. ctrl+b is a much nicer default.
The status bar displays key information to the user. Do you think people would prefer a video player with no play/pause, volume, seekbar or one with them? The interface is the most important part of any application.
The latter reason is why Helix is slow in doing similar to vim, I think, despite being far more consistently designed and saner defaults. Everyone knows vim exists.
I've still never switched from screen, personally, though I'm only a light user.