Why: The sidebar has useful features that you might want to access when you don't need other tabs(as it is the main reason a sidebar is more useful than having another tab), which hides when you enter fullscreen.

Sidenote: For me, i prefer having a page in fullscreen when for example reading manga but i wish i could still have access to the music sidebar, as the sidebar is an ease of access feature this sounded like a reasonable improvement in my opinion.