The juubi wasn't in final form at any point when it was sealed, plus it is still missing half of kurama which is arguably the most powerful Bijuu. Also the ten tails took a huge whooping during the fight with Naruto, that has to have had some effect. Beyond that, Tobi stated he used a large portion of its chakra making zetsu just prior to starting the war. Dunno how long it takes to get that chakra back but that probably has played a role too. IDK I think that if SO6P didn't fight it, its not cause it is weak, but because he is strong.