The monument was built as a nationalist anachronism, along the lines of the Victorian monument to Boudicca, and Napoleon III's excavation of the (supposed) site of the Battle of Alesia. There was no surviving folk tradition of so-called resistance to the Romans, not merely because of the great antiquity, but because Arminius' entire people and culture had by that time been covered by many other coats of paint.
Wasn’t familiar with the excavation by Napoleon III. Thanks for the tip. I’ll dig (pun intended) into that. Thanks SunkenCiv.