Well, there are attendees who have to book flights, apply for visa, ask their boss for holidays, coordinate their 2014 agenda with other events or just like to plan in advance.
Also, opening the registration early ensures a certain planning reliability for the host.
But fortunately, you are not forced to register on the 13th of January. Estrel and other lodgings in Berlin offer enough beds for everybody, you should find sometghing even if you register in July