There are many responses to that question.

First, the question is invalid on multiple counts.

• You do not need some specific threshold of witnesses. The minimum requirement is one. That requirement is met.
• The question introduces an irrelevant condition. It doesn't matter if it was "widespread" or not nor does it matter what the meaning of that is. All that matters is that it was happening. If it happened you now have to establish how much it happened, specifically if it was enough to sway the result.
• At this point interrogating the witness starts to become futile. You need to start issuing warrants and investigating the ballots. You can only achieve so much sitting there each trying to argue your cases. You can't solve this with endless conversation and argument.
• More people have come forward who have witnessed mishandling, malfeasance, irregularities or suspicious activity.

Witness intimidation is an obvious reason. Much of the fraud that happens is specifically going to happen when they think witnesses aren't around. Most witnesses present when it's happening are liable. You would be getting confessions, not witnesses. Sad though it might be witnesses don't always step forward simply because either they don't care or have other things they would rather attended to.

Only a percent, maybe a little more or less of votes need to be illegal to flip the result. When you have people handling ballots sometimes you have one or two people in a position to allow through thousands or tens of thousands each.