The reason is that quickview is tied to the other pane, so that pane is kept visible. This behaviour dates back when one couldn't focus a quickview and enter view mode, so maybe Ctrl+W o could be permitted in this case with the other pane being shown automatically on exiting view mode.
By the way, there is also Ctrl+W | to maximize a window. "explore" view (after e) can also be used in the only mode.