Is this because MS wants people to just open new windows of Explorer (hence the name, Windows), or is it because of a silly patent by Apple or someone that prevents them from bringing tabs to the Explorer?
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.