Originally Posted by Emissary of Justice
It wasn't that complicated.
"His passion and intensity permeate throughout the blade... you could say... the blade carries his very essence."
Therefore, it's only natural that it would fail Obito against Naruto. Kishi basically pulled the Goku/Captain Ginyu bodyswitch fiasco where Ginyu couldn't properly wield Goku's power initially because he was evil.
I thought that as well but then I started to think, "Where did the sword come from?". As I thought about it I realized that the blade itself must be based on physical chakra much like sasanoo's sword. The shield also relies on that same chakra as it basically acts as both a sponge and a barrier. The only problem is that it can only absorb so much punishment until it breaks. It also probably takes a considerable sum of chakra just to create a barrier system that powerful. Then there is also the cost requirements that Obito must have from just trying to get the tree ready. In essence he had two things going at once. He lost the shield and lost his focus for just a second while try to get the tree ready. This disrupted his control of his chakra which coincidentally controls the sword. The sword as a result lost its necessary density and sasanoo's blade easily blasted through it. While the idea of friendship and butterflies does sound nice, I think the plausible theory of loss of chakra control makes a little more sense. After all he also had kakashi playing around in his noggin so it makes sense that sooner or later Obito would fuck up. No one can be perfect forever no matter how good they are.