It is also, in practice, actively maintained and gets bugfixes. It turns out there are a metric buttload of obscure corner cases that do need to work right.
Which btw was my problem with it when using st. Corner cases like using nano under ssh just plainly did not work. Or copying stuff from the terminal that got linewrapped, if I get that wrap as a linebreak in the copy buffer it destroys the password I just copied, making it hard to use pass.
Here's the changelog: http://invisible-island.net/xterm/xterm.log.html
xterm is addressing a vastly broader audience than st.