Why are there two distinct political ideologies?

[J]

haha. It's interesting that you should bring this up because we were just talking about this in my comparative politics class.
Yes, historically, the United States (and I'm assuming you're referring to the U.S.) has had a two party system. But I don't think we're polarized. Not at all.

So, I'll give you two answers. One being why I think we've had two party systems and why we're NOT polarized.

The United States is kind of weird in that its political culture doesn't question its regime. And by political culture, I mean all the values and ideas our population has towards authority. While we may question certain policies and actions, have you ever heard of somebody trying to protest against the Constitution? It's just unheard of.
After so much time, we've grown to accept that regime which has endured now for over 200 years. And a two party system has always been associated with that.

I mean, you have to think about it, we've had republicans and democrats ever since the end of the Civil War. The only thing that has changed is what they stand for. Those parties are as American as apple pie.

Third parties exist, don't get me wrong, and they have a decent amount of followers, but they do not hold the degree of authority that the Republicans and the Democrats have.

Now, on to the other issue. Polarization. This is precisely what we were talking about today in class. Current statistics have been showing that party identification has been waning ever since the 2000 election.
In 2000, you sort of saw it. Since then, it has really gone up though. Currently, we have the largest amount of undecided voters in... ever. If this continues and it becomes a trend, then it's very likely that we'll see a realignment of dealignment of parties.