This may have historical roots: the national supervisory authorities, unlike the ECB or the ESCB, are not Community law bodies and do not have a European mandate enshrined in the Treaty. However, this entails some limitation in the ability of the ECB and the ESCB to perform their tasks.
One has to take into account a series of facts that make this problem less acute. First, a number of central banks in the euro area perform supervisory functions and therefore have easy access to supervisory information.