Worlds can get "full", and Hyperion is consistently the busiest. It could be that it will only open for new characters once a significant number of people transfer off of Hyperion or choose not to subscribe.

Even then, allowing new characters may not last long. After the big maintenance where SE increased server capacity 50%, most character creation restrictions were removed. But SE saw so many new characters being created so quickly on many worlds that they had to put restrictions back in place after only a matter of hours.