I can only offer you must best guess as I don't work for Microsoft. The modern UI framework that you use to create Windows Store apps cannot create a treeview

I can only offer you must best guess as I don't work for Microsoft. The modern UI framework that you use to create Windows Store apps cannot create a treeview suitable for an explorer type of application. Since Windows 8 how many apps have you seen that features a treeview with tons of nodes? Zero? So my guess it that the visual guidelines makes it impossible to modernize the UI of explorer.