Why? Because they lie and/or refuse to answer questions.
Example one: McCain’s senior advisor Steve Schmidt was quoted in today Wall Street Journal about whether Sarah Palin had been fully vetted:
This vetting controversy is a faux media scandal designed to destroy the first female Republican nominee for vice president of the United States.
That is nonsense, a blatant lie. Wondering how a presidential candidate made a giant decision (his choice of a VP) is a totally legitimate line of questioning. Even if you support McCain and Palin, you might wonder “did he really know everything? Did he rush into that pick?” To claim that it’s a “faux media scandal” is ridiculous. That is what journalists are supposed to do: ask tough questions.
Example two: this clip from CNN, which has gotten a fair amount of buzz. In it, CNN’s Campbell Brown asks McCain spokesman Tucker Bounds some simple and legitimate questions about Sarah Palin. He ducks and weaves, refusing to answer the questions, and then attacks the journalist for “belittling” Palin. Avoiding questions and sliming people makes the average voter completely tune out these spinmeisters.
Thoughtbasket 
How did the GOP get so good at spin? Do they train people? Do they have spin school? And why does the mainstream media mostly just accept it and not, as you state, ask the tough questions?
Oh, they have a school, alright. The right has been systematically investing in three key areas: 1) ideas; 2) media distribution channels; and 3) leaders. After 30-40 years, it has become the right-wing machine. Bill Moyers made a great documentary called teh K Street Project that illustrates these principles.
Our side has started to respond after 2004 with the Democracy Alliance, a group of donors who are now, over the long-term, looking to bolster and deepen our idea generation, media channels and movement leaders. Rob Stein’s “powerpoint” was the blueprint for this. http://www.sourcewatch.org/index.php?title=Democracy_Alliance