"For president" vs "for presidency"

Why is this statement correct Someone for president 2012 and not Someone for presidency 2012?

EDIT: So far my understanding is that neither of these statements are grammatically wrong. However, at this moment Google finds almost 2 million results for "Obama for president 2008" whereas the number for "Obama for presidency 2008" is only 40.

Usually we think of "president" as a job description. For example if I am choosing someone, I might say

Hamid is the right man for this job

The word "presidency" is much less common in English, so while I don't think "Obama for presidency" is actually wrong, it's so unlikely in practice that it sounds odd.

Presidency is basically "the office" or "the term":

the office of president
the term during which a president holds office

The United States presidential line of succession article employs the word presidency when listing the different criteria needed to apply or qualify for the presidency.

But you definitively run for (being) president.
(the notion of "election" is not part of a "presidency", while a president is "an elected official". You are running to be elected)

"So-and-so for president" is simply the traditional way to do this. We have a set usage of "elect s.o. for position title", where position title is the title of the job, with the meaning you are intending. It is not some other description of the position, simply because that is not how the phrase is traditionally constructed. There aren't syntactic or morphological rules that force this to be the case -- it is decided on semantic/pragmatic grounds.

We do the same for any type of position (items with preceding question mark = awkward at best):

• John for mayor / ?mayorship
• ("and the Emmy goes to...") John for best actor / ?best acting
• John for father-of-the-year / ?fatherhood-of-the-year