I don't know for sure, but I would guess the McCain piece had lies in it and was rejected on that basis. Most of the pro-war op-ed pieces that I've read do.
As for saying the New York Times is liberal....
They are right smack in the middle of mainstream. The New York Times was the biggest cheerleader in the leadup to the Iraq War, while the real liberals were out protesting. Want me to post links to the many dozens of "Saddam has WMDs" articles printed in the New York Times during 2002? They are not the slightest bit liberal, and neither is Obama for that matter.