Partly because the Greek state forced them to buy Greek treasury bills (until the ECB put a stop to it). They are worthless at the current CCC- rating. That's

Partly because the Greek state forced them to buy Greek treasury bills (until the ECB put a stop to it). They are worthless at the current CCC- rating. That's one of the reasons why they are most likely insolvent. They are illiquid (have no cash money) because too much cash money has been withdrawn (and the ECB stopped raising the Emergency Liquidity Assistance debt ceiling at 89G€). If they are actually insolvent, then it was most likely illegal for the ECB to continue the ELA for so long, but they (along with everybody else but the Greeks) did their utmost to keep the situation from getting worse.