The ball shot way out there. The Steeler was running for it, but pulled up when the official in that area came pointing at the ground.
If it had been closer, and scooped up, the red flag may have come out, but the ball was left all alone by the defender...poor choice on his part.
Not a poor choice at all, once the ref blew his whistle and pointed to the ground the play was done, nothing about a red flag could have changed that. Once play is ruled over you can't challenge to get anything that occured after that point, because they know players stop when the whistle blows and it would be unfair (and possibly unsafe) to reward a player for not stopping when the refs said the play was over.
You can't challenge a "no-fumble" call if the whistle has blown.