Because he's the only panda that infringes on copyrights and they want to reduce the possibility of getting sued?
Actually, that couldn't be true, in japan you have to prosecute all the copyright infringers or you can't prosecute any. If they try to take you to court, all you have to do is present a document that is older than your violation and say "hey, they didn't sue these
guys" and you can get off the hook. Well, theoretically...
But seriously, it has to be related to the fact that that panda is the only one that is a media icon. I imagine that they were trying to build up tension by revealing all the other pandas and then make you wait agonizingly for the last one to be revealed. You get very excited trying to guess what it is and when it will be revealed, so it turns out to be a great marketing tool. Or just a great cinematic device.