Because they prefer to lend the cash to the ECB at 0.25% than to the "real economy".

But I repeat myself

We're in a liquidity crisis and banks want to be in an all-cash asset position.

I wonder what would happen if the ECB set the deposit rate (remuneration on excess reserves) to a negative nominal rate.

There are three stories about the euro crisis: the Republican story, the German story, and the truth. -- Paul Krugman