Because something that gets lost in the two party system is that, at its core, there’s the conservative side trying to make things best for their majority group and then there’s a dozen different types of liberals trying to make the world better in their direction. One side wants to revert back, one “side” is pulling in a hundred different directions. Should we tackle education first? Financial responsibility? Human rights? Social safety nets? Pollution? Climate change? National interest? Global interest? There’s so many different topics that liberals want to progress forward in an order each individual determines. It’s not a single liberal force, it’s a hundred forces in a trench coat. Conservatives (of any country or topic) are some kind of incumbent majority/dominating demographic while the alternatives all fall under “liberal”. So the only thing that consistently unifies the democrats is what they don’t like