And here's the reason why:
How Freeview Works
How Freeview works is that there is only a certain amount of 'space' that it has, which in turn determins how many channels can 'fit', and thus how many can broadcast. At this current time, ABC1 only have aquired space to broadcast from 6am - 6pm, and with space currently completely full, there isn't alot they can do, until more space becomes avaliable - which is entirely dependable on the Freeview consortium, technological advancements in compression technology, or if a channel leaves Freeview for whatever reason. But for now, ABC1 are actively looking and waiting for more space to become avaliable, but until then, Freeview viewers will have to be satisfied with ABC1 being between 6am - 6pm.