No one had ever figured it out, really.
Oh, sure, he told Kakashi that it had been because the man had let Rin die (though he denied it with some twisted, nonsensical wordplay soon after).
Sure, he told Madara it was because the legend had saved his life (though they both knew it was a lie, in that particular case).
Naruto probably thought it was because he was misguided and traumatized (admittedly, it probably had some effect), and could even still be saved.
Zetsu was the only one that knew the truth, and he wasn't even there.
The second Obito became the Jinchuuriki of the Juubi, he whipped out a mirror and punched the air.
"Yes! Best mask ever!"
A/N: No. No, this does not make any sense. I don't care. All I know is that the first thing I thought of upon seeing Jinchuuriki!Obito was that he had gotten himself another mask. Like… his fourth, or fifth.
I mean, he just really like wearing masks, right?
The goggles (not really a mask, though they're a bit of a precursor).
Spiral Zetsu (not a mask either, but… I don't even know).
The stripy mask from the Kyuubi attack.
The most famous and popular mask, the Orange Spiral, aka: the Tobi Mask.
The Sharrinegan mask.
And now this.
TELL ME IT DOESN'T MEAN SOMETHING.