I think people tend to vote for many different reasons, but it the end, it's very often the same old song. Socialists vs Republicans here, CDU vs SPD in Germany, Lavour vs Tories in UK, Democrats vs Republicans in USA (see Hubcap, I didn't forget you ).
Of course, it happens that a " non conventional " party ever makes its way to succeed and reach the power. We recently had Syriza in Greece, Podemos in Spain, and years ago I remember Pim Fortuyn list in Netherlands. These made their way through specific condictions, hard living conditions in Greece and at a lesser extent in Spain, Pim Fortuyn's assassination probably " helped " for the victory of his party.
But when " everything's okay ", what makes that people always tend to vote for the same old parties ?
I guess part of it is tradition, When I was 18, my grandpa told me " In the family, we always voted for the communists, you have to do so ". My mum still does I think, not because she listens to what french communist party says, or for its ideas, but because that's just the way it is.
I guess some vote for a charismatic leader, which can twist the borders of the traditional parties, I think that's what happened with Nicolas Sarkozy. Energy, all that stuff, a way to speak that was " closer " to the way common people speak.
A tendency I feel these past years, and I felt that's what happened with Nicolas Sarkozy, is that some people didn't vote for some ideas, but they voted against the person, there was some kind of " Everyone but not him ". I guess any socialist opponent would have won (still, Front National would have lost).
Another one is to vote " for a single idea " inside a political programme. My dad vote for Front National, for one single thing : immigration (islamic). When we manage to talk about this without argueing, he acknowledges that there's NOTHING good in their economic/society program (leave Europe, set back death sentence, come back to franc currency ...), but still, he votes for them every time for ONE thing : immigration. I guess it might be the same, let's say in USA about the right to bear arms from the 2nd amendment, not matter how excellent or good the political programme could be, some people would get stuck on that single arms thing without even listening to what else is in the programme.
Finally, some vote for the political programme, that's what I do. There're some good ideas (and bad ones !) on both sides, and some candidates dare to try to find a " middle " or even " different " way, but they never even get an average success. One could argue, and I agree, that if all really applied their election political programme, maybe it would be different (but even then, I'm not sure).
Hubcap Special (and american MMO-Champion fellows) :
More than anywhere else (or maybe we simply never hear about it from here), is there something else than Republicans/Democrats that could have an average chance to win some presidential election in USA ? I always tend to remember those like Ross Perot, or Ralph Nader in USA : where they credible in any way but still had absolutly no chance to win or, let's say have 20 % of the votes (far from a win, but still interesting) ? Why is it so, simply that people didn't support them, or is it the " presidential election system " that makes that it's not possible ? Are there some deputies/senators who're not part or one of these 2 parties (could google, but as I'm talking about it, putting it there) ? Heard about Tea Party, but don't have much to say about it, looks like a total anti-federal right-winged thing from what I've read.