Plenty of reasons: Habits. Existing scripts. Muscle-memory. GNU/GPL over non-GNU/GPL. To mention a few.

I've honestly not found a single feature in tmux which I need which isn't already covered (and imprinted in finger-memory) by GNU screen.

On the flip side, I'd ask: What reason is there for a seasoned GNU screen user to bother switching from something proven, and which works?

These reasons are valid, but I'm more interested in technical reasons.

I switched to tmux when I needed to launch a lot of simultaneous jobs, categorized in three sessions (starting in "active", moving to "success" or "fail" when finished). I couldn't achieve it in screen at the time. The CLI interface of tmux is not very intuitive and I think somewhat inconsistent (maybe a bit like git?), but at least I could achieve what I wanted without major compromises.

After a quick google search my impression is that running a shell command in a particular GNU screen window is still painful.

> These reasons are valid, but I'm more interested in technical reasons.

Sure. And let's go full circle while we're at it: GNU screen supports connecting directly to a serial device :)

Last I checked tmux doesn't.

Other reasons have been listed already, but if you're an iTerm2 user, another strong reason to use tmux over screen is the fact tmux can use "native windows" in macOS and you have deep integrations between the two. It's especially useful with remote tmux sessions.