That's because it wasn't detected by anyone before impact. 1998 OR2 is a known asteroid and I have independently tracked it. Here is my astrometric data and orbit determination.
[link to drive.google.com (secure)]
They couldn't hide this information even if they wanted to. People like me have independently tracked it. You can't hide the information on a known asteroid.

This thread claims that they did that and that they suddenly changed the miss distance on a very well-known and very well-tracked asteroid at the last possible minute. That isn't plausible at all. If they had lied about how much it was going to miss by we would have already known that because of independent observations. There's a difference between an asteroid striking us that was never detected and an asteroid striking us that was detected but they somehow covered up the true close approach distance.