The footer copyright declaration at the bottom of the page
is in lower-case (completely). I want to know if there is some particular reason for this? Why is it not Titled-Case or Sentenced-case?
As Jeff says here: (in a highly unpopular response)
It's not a complete sentence, so it doesn't need to be capitalized. We don't capitalize the tabs on the user page, or the homepage, for example
Consider the copyright declaration:
site design / logo © 2013 stack exchange inc;
That's indeed not a full sentence. It's not "This site was designed by...". As a declaration only, it does not get treated as a sentence.
While I personally does not agree with Jeff reasoning, that's most likely the reason.