I'd think it is because the name "Wikileaks" has higher visibility and meaning to the typical Australian voter than "Pirate Party". Despite common ground, they

I'd think it is because the name "Wikileaks" has higher visibility and meaning to the typical Australian voter than "Pirate Party". Despite common ground, they also have a different highest priority, Wikileaks being transparency, Pirate Party's being copyright reform. Separate parties will avoid tension due to different priorities. Having two parties is not too big a deal in Australia's preferential voting system, as each party will probably pick up the other's preferences, meaning their votes will get transferred if they get knocked out of the contest.