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.
The political preconditions for single banking supervision were only born out of necessity as a result of the escalation of the financial crisis. On account of its independence, expertise and market knowledge, the ECB proved in practice to be the only possible location where single supervision could be established. The ECB will conduct banking supervision in cooperation with national supervisors.