Screen still supports RS-232 input, so there are some extra use cases it covers, that matter not to 99% of us.

Screen is GNU licensed, tmux is BSD licensed, again, mattering to probably 1% of us.

The package for tmux is about 1/2 the size of that for Screen (Debian Sid, amd64), while installed, it's 2/3 the size.  Possibly smaller in memory footprint; that's a difference of more practical value.

On modern systems, that difference may not much matter either.

I used to use Screen, shifted to tmux perhaps 8 or so years ago, and haven't had any complaints.

I think it's a bit easier to write scripts to manipulate tmux environments (which is not unlike what Giles noted about the scripting language of Screen).

There's also a likelihood that there are more people still working on tmux.

None of these factors point at dominant arguments, just at small possible advantages.