It is up to the bounty owner to decide what to do about the bounty.
Sometimes they simply forget about it. You can't do anything about that. And as pointed out in comments: there is an additional grace period. Maybe the bounty owner simply wants to wait for the last minute before awarding the bounty. There is always a chance that another, better answer comes in. But it looks like the OP of that question just missed to award it.
It is sometimes helpful to write a polite comment to "not forget about awarding the bounty", but that is about it.
And yes, that leads to the fact that sometimes the bounty isn't awarded at all, because forgotten. But the system is designed like that on purpose (for answers given before the bounty was established).
And yes, that is a very annoying feeling. Happened to me twice that my accepted answer missed a bounty because "forgotten". Worst case, take it as one of those events in life that allow you to grow your personality.