It was quite possible that the day the Mangekyo Sharingan had appeared had been the true fall of the Uchiha clan. With the knowledge of how the Mangekyo manifested in the forefront of everyone's minds, poisoning them by sowing discord and distrust, knowledge of how the Sharingan itself appeared began to fall by the wayside, and was soon forgotten despite all of the evidence that cropped up in later generations.
While the Uchiha cursed Kakashi for taking Obito's exceedingly powerful late-blooming Sharingan, they never bothered to care how he had gotten it or how it had formed the day that Obito had turned around and gone back for a beloved comrade.
While everyone praised Itachi for his early activation of his Sharingan, nobody either knew or remembered that it had formed when the young boy had gone out onto a battlefield in search of his parents.
While Sasuke had initially gained and lost his Sharingan on the night of the Massacre, it was only when he had been fighting alongside Naruto in a cage of ice mirrors that he regained it again despite the number of fights he had picked with stronger opponents in hopes of gaining it and growing stronger.
You see, while the clan remembered that the fear for one's life had been a requirement for gaining the Sharingan, they had forgotten one part of the formula. It wasn't merely fear for one's life that was required, it was the fear for one's life coupled with the desire to protect someone that was close to them that caused the eyes to manifest. No matter how many times a family member would have tried to scare a child in order to get their Sharingan to work early, it wouldn't have worked because, no Uchiha ever got their Sharingan when they were fighting for themselves alone.
