Acquired from Yahoo, in part because all day long when they refer to themselves (the company) they write or type "WP".
WP.com [WP.com]
Necessity or vanity?
Wisdom or "just because we can"?
BTW, WordPress was already #2 for WP on Google. I remember that from when I used to want to visit the Washington Post website.
Even so, I think sites that get visited all the time are best off with the shortest domains. Although most people use bookmarks for sites they frequent.
p/g