1) They support Amnesty even if they don't admit it.
2) They support big government health care administration.
3) They have untenable liberal positions on social issues guaranteed to drive the off conservative base.
4) They have a record that stands against conservative principles that will guarantee a Hillary presidency due to lack of turnout.
Which is the only reason they get vast amounts of air time versus qualified conservatives such as Fred Thompson and Duncan Hunter, who are noticeably "blacked out" by the media. Particularly Fred, who is either ignored or attacked in a concerted effort to paint him as "Lazy" despite his high polling numbers in LARGE conservative states.
Oh, I should toss McCain in there with Rudy, Romney and Huckabee. McCain positions don't matter as much as the fact that he's an unstable nutjob who collaborates with the other side so often they need to put up a crossing sign in the senate.