I can understand why he doesn't.
The average sunni has been thought since childhood that the sahabah were angels walking on earth. The more ignorant the area you come from is, the higher you elevate the sahabah.
Once you start talking about these events. Their rosy image gets shattered and they can potentially start having doubts about Islam.
I don't know what you're talking about, the average sunni does not have an image of angels walking on earth بارك الله فيك. I've never met a Sunni who doesn't know about the wars that happened, the assassination of Uthman رضي الله عنه, that some of the companions sinned. How many Muslims do you know don't know the story of the Companion who used to get drunk? Heck, even Shaykh Yasir discussed how Wahshi رضي الله عنه was known to sin even up until his death. I don't know of any Sunni Muslim that has that kind of view towards the Companions بارك الله فيك.
The reality is, he did a single lecture on Karbalaa many moons ago. For the past five years or so, he has been concentrating on the Seerah, and the reason he mentioned this topic was because (as he mentions in the beginning of the lecture) he felt it was directly related to the Seerah since it involved issue surrounding the death of the Messenger صلى الله عليه وسلم.
There is no reason to think that if he keeps going with lectures on the Khulafaa ar-Rashidoon that he wouldn't cover those issues.