11th November 2014, 07:56 PM
From my very limited experience and intellect I'd have thought it's a mix of: a dramatic change in the economy post-Roman influence leading to the demise of the commercial potter and the reintroduction of more 'primitive', locally made pot from a diminished skill-base, which also may have led to the seeking of alternative materials (you can boil water in any material that holds together, and holds water obviously, above 100*C...); a change in fashion; and as noted above and related to the first part, the confusion between the pot of that period and the pot made under similar circumstances in pre-Roman times. Or they had a good refuse collection service...
I reserve the right to change my mind. It's called learning.