It was proved that the officer had the right to use deadly force when Brown was charging at him, yet people still use it as an example of unjustified police violence; for example in this article (but there are others as well).
I'm not sure that it is being brought up that often.
But there are at least three reasons that I could think of that it might be:
An example of this line of thinking is this Slate article by Jamelle Bouie:
[Wilsons] account, given one month after the shooting, fits the facts of the case [...] But the fact that it’s possible doesn’t make it believable [...]
It’s the fear that’s most striking. Wilson was trained, armed, and empowered with the force of law. At almost any point in his confrontation with Brown, he could have called for backup and won control of the situation. But, he says, he was too gripped with terror to do anything but shoot. [...]
Maybe Wilson was an ordinary police officer with all the baggage it carries [link to a paper showing "implicit race biases in the decision to shoot potentially hostile targets in a multiethnic context"]. Maybe, like many of his peers on the Ferguson police force, he was hard on black teenagers. Maybe, like many Americans, he was a little afraid of them. And maybe all of this—his fear, his bias, and his training—met Michael Brown and combined to create tragedy.
Jonathan Capehart argues in the Washington Post that the narrative persisted because it fits the pattern:
In fact, the false Ferguson narrative stuck because of concern over a distressing pattern of other police killings of unarmed African American men and boys around the time of Brown’s death. [He goes on listing the Eric Garner, John Crawford III, Levar Jones, and Tamir Rice shootings.]