It is for the governments of the euro area, which together form the Eurogroup, to decide whether they wish to lend to the Greek government and under what conditions, not for the ECB. In 2012 Mario Draghi said he would do whatever it takes to save the euro, but within the ECB’s mandate.
The ECB is an institution that has powerful instruments, but it can only use them within the narrow scope that has been set for it by the treaties, under the supervision of the European Parliament and the Court of Justice of the European Union. Respecting the law is fundamental to respecting democracy.