Obito's weakness to Senjutsu was revealed quite a few chapters ago and for some time Naruto and Minato were using Senjutsu against Obito to weaken him. But why wasn't Sasuke using this earlier and before Obito summoned the God Tree?
Well, to start off, they had bigger problems at the time, such as stopping the 4 bijuu bombs from killing everyone inside that barrier. After that, Naruto was showcasing his powers.
But I'm pretty sure Sasuke was considering about the senjutsu power already. Remember when they were all trapped inside that barrier and Jugo asked Sasuke about getting them out of there using the summoning jutsu? Sasuke said he would only bring Naruto and Jugo.
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Why only those two: because they're the only ones with senjutsu.