Because aria-expanded is supported on these roles:

• button
• combobox
• document
• link
• menuitem
• section
• sectionhead
• window

The full set of roles it is supported on is:

• alert
• alertdialog
• article
• banner
• cell
• columnheader
• complementary
• contentinfo
• definition
• dialog
• directory
• feed
• figure
• form
• grid
• gridcell
• group
• heading
• img
• landmark
• list
• listbox
• listitem
• log
• main
• marquee
• math
• menu
• menubar
• menuitemcheckbox
• menuitemradio
• navigation
• note
• progressbar
• radiogroup
• region
• row
• rowheader
• search
• select
• status
• tab
• table
• tabpanel
• term
• timer
• toolbar
• tooltip
• tree
• treegrid
• treeitem

Is an alertdialog really expandable? What about a progressbar? And I'm not an author, but what is the advantage of making main expandable? Let's sanity-check these.