Perhaps it was the colder northern climates, or perhaps Romans had fashion fads too, either way the tunica exteriodum reached the ankles by 100 AD. Furthermore this Roman Tunic gained a new name, the Caracalla. From 200 AD almost everyone wore the caracalla.
The line drawing shown right shows a Roman slave in a short tunic perfect for working easily. The dark tunic in the corner was to become the foundation garment of basic clothing for hundreds of years.