Notifying fans on Twitter (in Japanese), Atlus said that downloads hit the planned maximum quantity. Apparently, the Catherine demo was planned as a limited release, but Atlus didn't tell anybody that there was a limit or what it was. Perhaps the company only budgeted so much money for the PSN fees required to serve the demo. All we can suggest is that you run to your PS3 right now and download every single Atlus demo. It won't help you experience Catherine, but it will prevent you from losing out on any more demos.
You can find a small, non-playable chunk of the demo -- the opening movie -- after the break.