The Maastricht Treaty did, in fact, state that the ECB shall “contribute to the smooth conduct of policies pursued by the competent authorities relating to the prudential supervision of credit institutions”, and that tasks concerning banking supervision could be conferred upon the ECB via a Council Regulation.
It must be remembered that at that stage large European countries (perhaps with the exception of Spain) had for decades not had much experience of serious banking crises. So, the question was not perceived as being a sufficiently burning issue. Big differences in national supervisory structures also played a role in why supervisory convergence did not make headway.