"Tobi" just explained that he could execute his plan (his ultimate objective) even with a bit of chakra from the Eight-Tails and Nine-Tails.
He's had the former since shit-I-don't-even-remember, and the latter since Kabuto revived Kinkaku and Ginkaku.
So why did he wait until now? It's no fun unless you're surrounded by enemies who'll try to stop you?
I think there is an actual reason for why he waited. Even though he could have revived the Ten Tails with their chakra, he admits that it will be weaker in this chapter because they're not the real thing.
He was probably hoping he could easily capture Naruto and Bee when he engaged them, but that obviously isn't happening.
So, he just said fuck it.