On low-traffic tags, a bad question may pick up one or two close votes, but simply hasn't been viewed by very many people with the close-vote privilege.
So, after 4 days, the close votes expire, and SO remains cluttered with poor questions.
Possible examples which all meet one or more close reasons:
Q#5037300, Q#6015594, Q#3315186, Q#4546611, Q#6543834, Q#6444752
Also a good chunk of these Google results probably could be closed.
Note that this phenomenon can also cause undo "churn" and fallback to the flagging system.
For example, see What Happened Here? Voted to Close 4 Days Ago, but No Close Votes Today? (and similar questions)...
A user notes a question that needs closing, votes to close it, and then later notices that the evil question has escaped justice.
The user then opens a question on Meta and/or fires off a few flags.
Only after lots of community involvement and moderator intervention is the bad question closed.
Close votes (and probably reopen votes) only expire after all of these conditions are met:
4 days have passed since the last close vote.
The question has received 100 views. (Note that the 6 example questions were viewed: 64, 54, 95, 121, 56, and 50 times, respectively, before this question was posted.)
The question was viewed by, say, 20 people who had the close-vote privilege.