Well, really, I see people saying that it's because he was weak and just a kid but... Kakashi is a dangerous ninja. Kabuto should've known that, and Obito would've been nice to see him meet Obito again, and be forced to fight him... would Kakashi have had the heart to strike him down?
Dan wasn't strong, but he was brought back.
Was it just because Kakashi and him had all the closure they needed?
I bet you thought this would be a Obito = Tobi thread, huh?