Obviously because he didn't do it he knew it's not worth it. That's why I'm speculating. We are talking about Naruto, so bad writing as such would hardly be out of the question, but we might as well humour the story and give it the benefit of doubt.

And sure he did it last time. Otherwise we wouldn't be seeing any Naruto jinchuuri action here if the suppression was permanent. There's only so much Tobi can do with haxoringan as long as Kyuubi is inside Naruto. Fortunately, as the thing is too powerful as it is.

Comparing to last time is half useless anyway since back then the opponent was Sasuke. They had no real intention to fight him seriously, let alone kill him. Because they want to save the dude. However, now we are talking about Tobi. I doubt even his own mother would want to save him anymore.