I'm curious as to why? Is it licensing issues?

I don't think tmux and screen have a feature disparity, since tmux is basically feature complete when compared to Screen.

I've used screen for 15 years, am happy with the functionality, and don't see the need (yet) to switch to tmux. Given that, I would rather that iTerm2 not use more system resources supporting a vterm multiplexer that I don't use. It's not going to keep me from using iTerm, though.

That, and screen is installed on pretty much everything I use. Tmux isn't.

You can modify tmux to use the same shortcuts as screen so that you can use the same muscle memory. I've been a heavy screen user for a while and recently switched to tmux. To me, tmux felt like a breath fresh air. That said, I spent the time (~hour) to convert my heavily modified screen environment over to tmux so I'm using pretty much the same shortcuts.

iTerm2 does not use "more system resources supporting a vterm multiplexer" if you don't use it's tmux functionality.

Same goes for most all well designed apps a lots of not well designed too: code pages are loaded into memory only when they are needed.

Why switch if screen does what you want? Especially when screen is available "everywhere". To me tmux is a solution to a non-existent problem.

my understanding is that tmux has a cleaner code base, is more easily configurable, and is actively maintained. by comparison, screen is said to have stagnated, and have a convoluted code base.

for the moment, i continue to use screen, for the same reasons stated elsewhere: i'm familiar with it, and it's available nearly everywhere. but i'm not averse to experimenting with tmux