Let me put it to you this way. In the early 20th century, the only way to win elected office in the South was to run as a Democrat. You had a lot of Democrats who essentially were just running as such to win. Many of them became the Blue dogs and some even jumped ship when the Republicans took over. When Republicans started winning though, a lot of people switched sides and became a Republican only because they wanted to win.
Political parties are too often just a rallying point for people with designs on power.
A lot of politicians are just opportunists. It easier to be a democrat opportunist than a republican.