Bunch of reasons.
Rules are murky around what is deemed an illegal strike. Different rules at different points in time. Apparently he also received advice from corner to throw the knee. Also you could argue he was anticipating Sterling was about to get up. See Anders vs Tim Williams. Finally, was it a case of a fighter losing and intentionally cheating to try and gain an upper hand? Or did it fall more into the category of "careless mistake"?
Foul =/= loss. Fouls happen all the time. Yan already received the worst possible in-fight punishment from this, from official warning to point deduction to no contest to disqualification. He received the worst of those four possibilities. If everyone was on the up and up, the fight woulda continued. It only shoulda been stopped if Sterling was unable to continue. I believe he was able to continue, and acted to earn a fake victory and get out of the fight. Had the knee been clearly legal, he would have acted in a totally different way. So that bullshit response from Aljo plays a part.
Third reason: Yan is better than Aljo. He was better in this fight, and he'd be favored in the rematch. This sport is about finding out who the best is. Unless you think it's obvious that Yan would just get disqualified in the rematch again, you have reason for it to be run back.
Running it back is clearly the correct decision.
