Because they switched to a new greeter in 20.10 compatible with Ayatana Indicators and there's no GUI to change those setting yet. Reasons are mentioned in the 20.10 release notes.
There's a reason interim releases aren't LTS. They're paving the way for the next LTS, introducing new features, one at a time and things aren't expected to be perfectly polished.
You can edit the Arctica Greeter settings with:
sudo pluma /usr/share/glib-2.0/schemas/30_ubuntu-mate.gschema.override
Don't screw up while editing things though. Once you've saved your edits, run:
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/