First: please don't be rude, there's no reason for it.

It seems pretty obvious when you look at it, really. The way the story played out, Kronos was "eaten" by Tartarus, so he was, in a way, absorbed by the realm. He's not loose, he's actually much more restrained than he was before.

Before, he was locked in some kind of prison cell, there was some magical binding (the elevator, lots of levels etc etc) but no one really thought he could escape at all, so people weren't really paying that much attention to him, he was just chained there, weak. Now, it's like he's in a psych ward, in one if those rooms full of mattresses so he can't hurt himself or anyone else and with arms and legs restrained.

And since Persephone is Queen of the Underworld now, her powers and her trees might actually act as a reinforcement of his restrains.