Because in the course of her history, only two parties came into dominance. There was a communist party too, but I think they weren't popular in a country where for most of her recent history, they put their trust in God, which is printed on their Greenback. There are no laws to stop anyone from forming another party. By all counts, USA is a democratic country. Mode of operation is Capitalistic. There is also freedom of the media, the right to choose/or not a religion, choice of sexual orientation, and another which makes it more democratic than others - the right to bear arms.
Cheers!
Not entirely true. The American Constitution is a rare piece of work, and very democratic. It requires for the bearing of arms, not to endorse hunting of deers or bears, but for the people to defend against a BAD government. FACT.
However, different states in USA have different laws and execution of the laws. Shit, USA is a coming together of 52 factions that written into its constitution, it allows for them to separate. FACT. There is no real freedom of the media. Large corporations are in control of the media and the country's destiny, not the people. The President is a puppet serving a shadowy crew that does not see the light of day, you do not know who the crew is, but they know who you're. The school is not allowed to bring religion into the classroom, by law. In this regard, Singapore is more democratic than the Yankees.
Abortion and sexual orientation are not legal for all states. Some states do not allow same sex marriage. Even the death penalty is not practiced in all states. Certain states are lax in the use of drugs, certain states no. The right to bear arms does not make a country democratic, but the right to kill with impunity does, but no country allows that, so there is no true democracy.
In other countries like Syria, Iran, Thailand, Taiwan, etc,etc people can also bear firearms, and they use them too, whether they've the right to bear arms or not. It's the ability to shoot and kill without being arrested that should determine if one is living in a democracy or not, but then again, true deomcracy in itself is an illusion, for it means the absence of a government.