Stack Exchange's aggregate traffic is public and available on Quantcast, but it may or may not be an accurate or up-to-date picture at any moment in time. It should, however, be accurate enough for the purposes of figuring out if a contest helped: it was used a lot on Gaming.SE during the Skyrim vs. Modern Warfare 3 promotion.
Much more detailed analytics are indeed available to SE employees and to moderators, but they're not available mostly for the reasons outlined in the answers to the MSO question, Why are stats not public information?.
First, cwallenpoole's answer:
If I might hazard some guesses:
This might have to do with the same reason that SE sites are "not about the rep". If exact numbers are used, that could lead to someone gaming the system, realizing that x and y conditions lead to condition z. In the context of a blog, this can lead to especially perverse aberrations.
It also could have to do with principles similar to those outlined in Tufte's Visual Explanations. The quantifiable, without appropriate contextualization can lead to grossly inaccurate conclusions. (you can see his discussion on pg. 36, if you care. A better example from a different book is the stock-market to sun-spot ratio chart at the beginning of Visual Display of Quantitative Information).
One rule to rule them all means both that you don't have to micro-manage what is and is not appropriate and you don't have the complaint, "But
<pronoun> gets to do it
Of course, these are more guesses as to/justification of the reasoning than they are the rationale themselves.
Then, Jeff Atwood's answer:
Pretty much what cwallenpoole said in his answer; in my mind, this is like arguing for full disclosure of people's salaries.
And anyway, what's the problem?
Those who need to know, can get the information.
Those who wish to know, can ask those that have it for a summary.
Just because you can get the information doesn't mean you need it. Information porn can be harmful. And having it can cause a distraction from the stuff that matters, just like real porn...
As you mentioned, they'll generally provide a 10,000-foot picture of those analytics upon request, though.