Why should someone vote for the Democrats when they will just throw us under the bus the moment it becomes convenient?
And because republicans are a unified block whose entire platform is to serve you for dinner. Their whole existence is dedicated to reversing any change.
You would like a third, fourth choice outside of Democrats and Republicans? Well, you would need a ranked-choice voting system for that. Remind me, who is pushing to extend that system of voting? And who is completely opposed to it and is even banning it wherever they can? Dare to answer?