Hi,
Thank you for asking the question.
Short answer: because of Wayland.
Longer answer
Wayland doesn’t support this feature. Wayland (instead of X11) is the future, and it’s the window manager (gnome-shell, or the one from Xfce, etc) which decides more things related to open new windows, where to place new windows on the screen, etc.
So… unfortunately the gedit feature you describe (with the workspaces awareness) has been removed.
BUT! It would be possible to bring the feature back. gedit still uses GTK 3, and GDK 3 (GDK is part of GTK) has X11-specific APIs with the workspaces etc.
I think that, if one day gedit is ported to GTK 4, the feature would no longer be possible (easily).
Also, how it was implemented in gedit, it used a X11 library directly instead of using the friendlier GDK 3 API.
Since gedit used (and needed to be linked against) the X11 library, gedit with Wayland relied on XWayland to work on a Wayland compositor. Now, when running gedit on Wayland, it’s a native app (“pure Wayland”).
By adding some GSettings keys and by using the GDK 3 APIs, the feature can be brought back for those still using X11, and still be a native Wayland app.
So, if you want, you can take an older version of gedit where the feature isn’t removed. After all, it’s free software
Read or search the NEWS file to know when that feature (related to Wayland) has been removed.
If someone is motivated, contributions are more than welcome to implement what I said. Or (easier), a branch can be maintained and rebased from time to time with the old implementation that relied on the X11 lib directly.
Edit: I know that Xfce uses X11 and not Wayland currently, a sentence above was not really clear about it (I was talking about window managers).