Quote from ThrikI think it's mostly down to people wanting to put their weight behind a party they think will actually win. It's like in the UK we have many parties as you'd expect, but it's pretty much Labour vs Conservatives at election day and has been for decades.Other parties are realistically going to take an awfully long time to gain anywhere near enough momentum to challenge either of the above, and probably not even in my lifetime, so I'm assuming most people choose to vote for their favourite of the two that stand a chance.
I think that most people are just "straight-republican" or "straight-democrat" and thats half the reason why it is the way it is. On our ballots the first two options are "all democrat" or "all republican"
Just shows you the way it is displayed to the voting public. Everyone is pretty much brought up on either it's one way or the other with little education on everything in-between, outside, or around.
Just my theory, I guess.