Because Viktor is a controlling bastard. He manipulates and controls people by manipulating and controlling information. Same way he lies to Selena to keep her under his control, same way he had lied and manipulated Marcus as well. In Evolution, we learn that Viktor banished Tanis due to the history Tanis was recording. We know its factual, but with Tanis gone, Viktor can change history.

>**Selene:** Andreas Tanis. He was the official historian of the Covens. He fell from favor after documenting what Viktor considered ***"malicious lies"***. Of course, as it turns out, he was probably telling the truth.

Very few immortals in the modern day are old enough to know the truth. Viktor, Kraven, Tanis, Marcus (arguably William as well), Lucian, Raze, and Amelia, the warrior turned princess who is killed on the train.

Keep in mind that even the events of Rise of the Lycans happen well well after Viktor becomes a Vampire and the 3 elders have started ***leapfrogging through time***.

He likely still believes that killing Marcus could in turn kill him, a bluff that Marcus used to keep himself and William alive. Having someone look into the legend could have problems for Viktor, which he very much does not want.