Page "Mellitus" Paragraph 7

Although Gregory had intended London to be the southern archbishopric for the island, Augustine never moved his episcopal see to London, and instead consecrated Mellitus as a plain bishop there.

After Augustine's death in 604, Canterbury continued to be the site of the southern archbishopric, and London remained a bishopric.

It may have been that the Kentish king did not wish greater episcopal authority to be exercised outside his own kingdom.