Because when the race is different by enough votes that those uncounted and problem votes won't matter. Meaning if you are showing 90% of the vote is counted and you are leading by 20,000 votes and they know the uncounted and problem ballots total up to 15,000, then even if every single one goes to the losing candidate, they still lose. So they call it. It is when the vote is close enough that those same ballots can pull the election one way or the other that they won't call a race for either until all are accounted for.