I've also wondered about this. One thing I considered is how many of the clan had not experienced a tramatic event required for three tomoe Sharingan activation until that night. Perhaps some were forced to witness the demise of their families, which would indicate an agenda to harvest fully mature sharingan. Certainly Danzo would be interested, but why Obito? There is also the Uchiha tablet to consider in this equation. I am not convinced that all has been revealed and if someone wanted to keep items on it hidden, eliminating all others capable of deciphering would be logical. We shall see.