Because they don’t believe in choice, freedom, or even democracy. They merely support the ideal and facilitate the illusion.

Anything that empowers voters to either dilute the two-party hegemony (where both parties are accountable to the same pool of donors) or elect party members that haven’t been carefully vetted by insiders is a threat to entrenched power structures. Adding roadblocks now ensures that transitions to better systems are made impossible via peaceful and lawful means.