I'm going to venture a guess that OP meant "Why can't we use Party Finder cross servers?"

As others have said, Duty Finder is separated by 'server groups' (for lack of a better term). Every 'group' has its own special name. Because I am on Hyperion, my DF group is known as Primal. I get matched up with people who are also from the Primal DF server group, and ONLY people from those servers. Server groups are there so that there is less straight on instances.