Why did Uninstall say some items couldn't be removed?
Short answer: Some items were in use or protected by macOS, so they stayed put; the screen lists which. Quit related apps or restart, then uninstall again.
One or more items were locked or protected — for example still in use, or protected by macOS — so they stayed where they were. The confirmation screen lists exactly which ones, and everything else went to the Trash as usual.
Quit any related apps or helpers, then try Uninstall again; a restart often releases files that were in use.