Part of it is that the Republicans are doing their absolute best to drive the scientists and scientifically-minded out of their party. The outgoing Congressman for my parents' district is one of the 6%- he's a Republican who was a tenured physics professor before he got involved in local politics and through a chain of events, ended up in Congress. My Dad's worked on multiple projects with the Member, and he knows Dad well enough that he'll be fairly open and honest with him.
And after they did their last tv show together this fall, they got to talking and the Congressman essentially said that he was becoming more and more certain all the time that his decision not to run again was the right one because the Republican Party that had been there when he started was pretty much gone and replaced by people who were so convinced that they were on the one true path that they'd utterly refuse to look at any evidence to the contrary, no matter how conclusive it was and how, while he'd always voted with his party 80-90% of the time, he was disgusted at the new standards of ideological purity that the current form of the party was insisting on and how if he had stayed, he was probably going to get himself branded a 'bad Republican'. He also couldn't stand the Republican nominee who was likely to (and did end up) replacing him- thought he had neither the experience nor the brains to do a competent job in looking after the interests of the people of his district.
And you can see his point in the bigger picture. The two loudest branches of the Republican party right now are the Christian fundamentalists who are actively hostile to a lot of major research going on in the biological sciences right now, and who also have issues with physicists and geologists who are inherently going to challenge the literal interpretation of the Genesis creation story, and the populists who regard intellectual curiousity and true high academic standards and academic success as indicators of liberal elitism.
I suspect that a good majority of the 30% in the poll that describe themselves as independents actually do hold a lot of core Republican values. However, they refuse to identify with a group that seems to want to drive them away from the party, which is understandable.
Why bang your head against the wall like that, or go where you are told you're not wanted?