The new behemoth?: the ECB and the financial supervision reforms during the Eurozone crisis

When it was created in 1999, the ECB did not have supervisory powers over the Eurozone banking sector. Since the financial crisis, it gained macro and micro prudential supervisory authority over the whole Eurozone banking sector. As this concentration of power is not riskfree and other policy alternatives existed, why were these competences delegated to the ECB in particular?