Read other letters about this article
Part of it is probably linguistic. The demons of talk radio have turned the term "liberal" into a pejorative, which people, unless they think, would not want to be associated with. If the surveys were to replace "liberal" with "progressive" and/or "conservative" with "unchanging" then the results might be totally different.
Language, after all, is very critical, which is why both sides of the abortion debate refuse to use the word "abortion." They are either pro (or anti) life or pro (or anti) choice.