Here's a question
Who decided that "red states" would be those which vote for Republicans and "blue states" those which vote for Democrats?
Wikipedia even notes:
"This system used in the United States of America is in stark contrast to the color system used in the vast majority of other nations. In most other parts of the world, blue represents right wing and conservative parties, while red represents left wing and socialist parties."
My bet is that's part of the point. Whenever the two colors began to be generally used in American election coverage, liberals probably felt that to have support for them labeled "red" would be rather like handing Edward McCarthy a posthumous victory. The United States spent half a century locked in potentially lethal stand-off with The Reds. Nobody in American politics, not even the Communists themselves, desires association with the color now.
So, when you think about it, it really is only fair: in order not to bias voters against the Democratic Party, best not to identify it with the people who embodied the logical progression of Democrat ideas.