That's a fair question - but the Democratic is currently the only legitimate political party in America.
So the tent is necessarily big. A more satisfying situation would be three parties: The Christo-Nationalist Party of America (in other words, the current GOP), which would eventually consist of people behind bars, the Corporate Party, as represented by centrists, and the Democratic Socialists.
Sadly, the Democratic Socialist Party would be tiny given the current makeup of the body politic. This is just the reality we live in today. The pendulum has swung far to the right over the last 40 years.
It's gonna take a lot of effort to push it the other way.
Nobody is really interested in taxing the rich or taking away corporate power. People talk a good game, but the vote counts reflect a grimmer truth.