[The Squatter of Amon Rûdh]

But of course, the Romans. They did, after all, forcibly remove the Druids from southern England.

And destroyed the Druidic sanctuary at Llyn Cerrig Bach on Anglesey (North-West Wales). When the Romans destroyed something it stayed destroyed.

This is why Tolkien's self-appointed task was so impossibly huge: all of the earlier Celtic myths that were remembered after the Romans left were carried into Wales when the Anglo-Saxons pushed the Celts out of what is now England, becoming Welsh rather than English tales. The Anglo-Saxons didn't start writing things down until after they were Christianised, by which time they had a very good reason to bowdlerise or ignore their pagan myths, which presumably survived in an oral tradition until some time after the Norman conquest. The Normans, however, just brought their borrowed culture over and established it as the ruling ideology, until none of the literate could or would remember the old stories. Tolkien could only guess at what had been by reference to known mythology from all over Europe, and from what little of real English myth was written down; but he did a good enough job for my, admittedly less cultivated, taste.