Because we conservatives had loyally supported whatever mediocrity the GOP put up since first nominating Poppy Bush in 1988, they imagined whomever they nominated we'd support.
With John MacCain they've discovered the lower limit beyond which we will not go.
McCain: Border security before path to citizenship
http://www.freerepublic.com/focus/f-news/2045605/posts
“But disappointing some at the Latino civil rights gathering, he said he stuck to his position that he only would consider a path to citizenship for illegal residents after he can prove to Americans that the border is secure. “