Ubuntu was released with GNOME 2.8, was shipping Unity as the default desktop at the time GNOME 3 was released, and only switched the default desktop to GNOME 3

Ubuntu was released with GNOME 2.8, was shipping Unity as the default desktop at the time GNOME 3 was released, and only switched the default desktop to GNOME 3.28 (configured to look more like Unity) in 2018.