You are off by an order of magnitude. There are only 4,516 (at this moment) eligible voters on Superuser. Supposing only 100 people voted in the primary then we've caught over 2% of the voting population of superuser.
It's very important to understand that nominations were only on a yellow banner for the nomination period for two days purposefully to avoid those people that don't come to superuser every day. If they aren't there often enough to see the banner, they aren't going to be there enough for moderation duties.
Similarly, the primaries are an opportunity for the most active users to narrow the field somewhat.
Voting will be open for only four days, but the banner should be up for all four days, and I expect you'll get a significantly higher turnout in that time period than the others.
Keep in mind that the mandatory (ie, couldn't be dismissed) notifications for all three periods of voting are present on the site for nearly 50% of 2.5 weeks. Yes, it's quite possible that many people will not be present at all during those two weeks, and it's possible that someone will only participate during the days where the notifications aren't present. They will be in the minority.
Further, I think people are putting too much importance on the moderator elections. They are not kings of the site. They influence direction only slightly. I'm really quite interested in how you expect another few days of voting, which would only capture a small fraction of additional votes, would actually change the outcome of the election, and why that particular change would be important. Are late voters going to choose significantly differently than those that were around and voted during the first four days?
The first few elections on Stackoverflow were open ended - they had no ending date, until they actually ended (surprise surprise!) and people were getting anxious to see the results. A week was too long. Unfortunately the ballot file given at the end of the last SO election doesn't have timestamps, so we can't see the voting trend, but it's my expectation that after 3-4 days the voting was very low.